T-54/55 series is the most-produced tank in history (100,000), replaced by the T-62, T-64, T-72, T-80 and T-90
T-55 - basic PAZ (Protivoatomnaya Zashchita) NBC ((nuclear, biological, and chemical) protection. The loader's DShK 1938/46 antiaircraft machine gun was removed (added again later). And lacks a turret dome ventilator.
T-55A - improved NBC (nuclear, biological, and chemical) protection, 12.7mm DShK 1938/46 antiaircraft machine gun was added again after 1970.
T-55 Enigma - T-55, Type 59, and Type 69 tanks used by Iraqi Brigade commanders had appliqué armour on turrets and hulls composed of 3 (hull) or 6 (turret) layers of local-designed NERA (Non-explosive reactive armor) enclosed in steel boxes, this composite armor named unofficially as "Haji". Every NERA layer consists of 3 sublayes: a 10mm thick aluminium plate, a 4mm thick natural rubber (as reactive element) then a 4.7mm thick high hardness steel plate. During the battle of Khafji (29 Jan ~ 1 Feb 1991), it is reported that this armor was successful at defeating several hits from MILAN missiles (which can knock out a T-72M1 frontward)
Type 59 - Chinese-produced version of T-54A.
Type 69 - Chinese second generation main battle tanks. Based on Type 59 (Chinese-produced version of T-54A) with technologies derived from T-62. (1974~present)
Tiran-5 - Israeli Modified T-55 with original 100 mm gun (Israel captured many T-54/T-55 from Egypt & Syria in 1967). It has two water cans are fitted to the rear of the turret, new fenders, a rounded stowage bin on the rear of the turret and pintle-mounted .30 cal M1919A4 Browning medium machine gun in front of the loader's hatch. Later fitted were the .50 cal M2 heavy machine gun over the barrel of the tank gun, extra gas cans, first aid box, ring around the loader's hatch for the .30 cal, new lights similar to the ones used in the M60 Patton, a folded stretcher on the left hand side of the vehicle and an infantry tank-telephone on the rear of the hull.
Tiran-5Sh (T-55S) - Israeli Modified T-55 with Sharir 105 mm gun. It also has ammunition racks modified to suit the 105mm ammunition, new communications equipment, modified commander's seat, new gunner's seat, azimuth indicator installed, driver's hatch modified so that it can opened from the outside, coaxial machine gun replaced by a 7.62mm Browning machine gun and the cupola mounted DShK 1938/46 antiaircraft heavy machine gun replaced by a 12.7 mm Browning heavy machine gun, new fire control, night vision equipment, electrical system, air-conditioning system, antennae mounts on the rear of the turret, exhaust outlet angled upwards, additional track stowage and fire-extinguishing system installed.