In 1991, during the first Gulf war, the coalition forces faced a new type of T-55 in service with the Iraqi armed forces. Locally designed, this tank featured an improved protection thanks to add-on armor blocks fitted to the hull sides, the glacis and the frontal arc of the turret. To compensate the weight on the front of the turret, some more blocks served as a counterweight at the rear of the turret.
The blocks were bolted to welded brackets. On some tanks, the blocks of the glacis were also hinged.
On the turret, 4 blocks were fitted to each side of the gun. The block above the driver station was mobile and could be hinged upward to enable the driver to access his seat or dismount. Due to the block weight, two large springs helped its handling.

For a long while, the type of armor remained unknown hence the nickname Enigma given to the tank. Some stated that the blocks were concrete blocks which was unlikely because on one hand of the excessive weight of such a solution and on the other hand of the poor protection capacity of this material against antitank projectiles. The study of the tanks captured at the end of the war showed the blocks were just steel casings filled with a sandwich of armor layers.

The name Enigma was given to any of the tanks fitted with this armor package whatever their type, T-55, T-54 or Type 59. It seems that these up-armored tanks served as command vehicles. Some were also fitted with a AA MG. Some vehicles are preserved in the US, in France and in Great-Britain.

The kits

The donor kit is the Tamiya T-55. It is currently the best kit available and it builds easily. The conversion is by Accurate Armour. It is not the only one as there are 5 in total (Verlinden, AEF Designs, Cromwell, Accurate Armour et Legend). Verlinden conversion was designed for the Esci T-55, AEF Designs and Cromwell's conversions are pretty hard to find and the Legend one was released 4 years after Accurate Armour's one.

The conversion comprises 83 light grey resin parts and 8 rather thick photoetched parts. The carving is overall good. Some side armor blocks show a prominent mold line. This put aside, the preparation work is limited to the removal of the pouring blocks.
The instructions come as 2 A4 sheets with color pictures of a model in progress. It really is advised to have real tank pictures as the instructions as sometimes insufficient to understand where to fit the parts.

The build

Having had the opportunity of seeing the tank of Mourmelon in France, I decided to build it. So some modifications will be necessarily made to the conversion.

The  hull

We have to start by cutting the sponsons according to the instructions.  Then the replacement part is set in place. The various holes in the glacis are filled. The armor blocks on the glacis are set on the right location with the help of the towing hooks. A small gap between the both blocks is normal. The small outer blocks are glued so their upper face sits horizontal. The hinges are not used.
The headlights protection is set after the resin lights are glued. The power lines are added. The convoy lights are moved to the small outer blocks. The PE brackets come from the Eduard sheet. The power lines are added as well as the conduit beside the driver's hatch.
The wiper fluid tank is provided by Accurate Armour. Hence we need to create the whole system missing on the Tamiya kit.
The side armor blocks are installed.
On the Mourmelon tank, the lower glacis is flat, so we need to sand the part D9 and fill the holes.

At the rear of the hull, the fuel tanks are replaced by the Accurate Armour parts. They are smaller certainly because some shrinkage issue during the mold release. They feature the fuel lines connections. We just have to make the lines. On the Enigma there is no front fuel tank which simplifies the fuel lines scheme.
On the left side, the resin exhaust is put in place. The metal cover is the Eduard one. The oil tank #19 is glued on top of it instead of the bin #24 to depict the Mourmelon tank. The latches of the bin #C are different so they are modified.
On the engine deck, the typical frame of the Russian T-55 must be represented. It is a major omission in the Tamiya model. Other small details are also added on and around the grilles.
On the rear deck, the cradles for the fuels tanks are not installed. We just have to set the welded brackets. These are done by sanding the Tamiya parts.
The brackets for the unditching log come from the Eduard sheet. They are modified to be true to life. Two additional towinf eyes are made from thick plastic card. A bolt head is added to each sprocket housing.

The turret

The tank of Mourmelon has an early type loader's hatch which is not in the Tamiya kit. It is reproduced from a Blast Models one.
Then the armor blocks have to be positioned while keeping a 8mm space from the bottom. It is advised to use a spacer. The blocks must be lined off the slots of the sight and the co-axial MG. Those willing to show the block above the driver in the raised position will have to study their documentation to figure out how to place the parts provided by Accurate Armour of which the lock. The extended springs #36 are too long by 2mm and must be shortened.
The gun mantlet is detailed. The IR light is located more backward. So the liaison arm with the gun must be adapted. The PE cover is set in place. The Accurate Armour parts are very thick and no template is provided. It is advised to heat the parts to help their shaping. This is also true for the tank commander light.
The resin hatch is more detailed than the Tamiya part. The episcopes are separate parts as well as the TC sight. To help the handling of the model, the hatch will be glued only after the painting step.
The counterweight blocks are glued to the frame #16. The right arm is too long by approximately 1mm. It must be shortened where the weld seam is depicted. A 8mm space with the turret bottom must also be kept.
4 small triangular back-up plates are present inside the arms on the Mourmelon tank.
The tie-downs are glued accordingly with the real tank.
To finish, the Armo barrel is set in place. It fits perfectly.

The painting

The base shade is a mix of XF-59 and XF-14 which is quite close to the XF-57. I wanted to depict a worn aspect without showing a scrap tank. I applied green chips with a sponge particularly on the edges.
Then, a filter of burnt sienna was applied over the whole tank followed by a wash of burnt umber. To tone down the rusty look, another black wash was finally applied.
Some variations of shades were created with MIG Gulf sand pigments. The rust stains were made by applying a small dot of burnt umber and brushing it immediately downward with a large brush.

References
Section Real Ones
Prime Portal
Mourmelon tank
Tank preserved in a US museum
SVSM
Bovington tank
Patton Museum tank
Iraqi T-55 PDF file

Index

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