NATO member Slovenia has sent Ukraine 28 M55S tanks, which appear quite modern and they are. But the M55S is a heavily modified T-55 tank that was designed in the 1950s and produced into the 1970s. In 1999 Slovenia completed the conversion of 30 T55 tanks into the much-upgraded M55S. The original 100mm cannon has been replaced with a western 105mm rifled gun. A new Israeli fire control system and explosive reactive armor was added. The suspension was upgraded and new tracks installed. A modern 600MP diesel engine replaced the original Soviet 580HP diesel. The M55S is similar to the Israeli T55 upgrade that produces the Ti67. The original T55 weighed 36 tons and the M55S upgrade created a 38-ton tank.
The T-54 was the first modern Russian tank and entered service in 1947, A major update, the T-55 entered service in 1959. About 100,000 T-54/55 tanks were produced between 1967 and 1979. Thousands, mainly late model T-55s, are still in use in active (kept ready to use) storage.
Ukraine found the M55S useful because it was a modern tank, although not as heavy (50 tons), armed (120/125mm gun) or mobile (much more powerful engine) as current Russian or Western (M1/Leopard 2) tanks. The M55S did not have an auto-loader and was not as vulnerable to destruction as Russian tanks introduced since the late 1960s. Without the autoloader, the T-55 had a four-man crew, which made it easier to maintain the vehicle and keep it operational. The auto-loader vulnerability was not realized until the Ukraine War because Russian tanks had never faced so many top-attack ATGMs (Anti-tank guided missiles) or so many modern tanks with 120-125mm guns. The Russians only realized this when over half the modern (auto-loader) Russian tanks used during the first few months of the Ukraine War were destroyed or abandoned. In effect, the auto-loader tanks were eight times more likely to be destroyed in combat than a modern Western tank. This means that the M55S is nearly as effective in combat as third-generation of Western tanks (Leopard I, M-60, Chieftain).
Slovenia received 40 modern military trucks and light armored vehicles from Germany. This exchange is common when East European NATO members send Ukraine Soviet era tanks that they are still using, or have in active storage. This includes some T-72s, which have a 125mm gun and auto-loader. The M55S is more useful than the T-72s, but the Ukrainians have learned how to use the T-72 more safely than the Russians did.