The "T" designation appears to change meanings. Pre-war, "T" meant simply "tankette" or small tank. Post-war, the "T" was a "test" or "prototype" unit.
As the USMC units were rushed in to combat, the "T" designation did not change to an "M" designation. Also, as both the USA and USMC were still using M-26 Pershings, the T-32/T-24 designations were probably kept to simplify things.
The Soviet "T" designation was for an active tank made by a certain factory receipe. Hence "KV" series, "T" series and "JS" series tanks. The "KV's" were made by Klimenti Vorosholev(sp) and the "JS (IL)" series were changed to Josef Stalin in honor of him.
Once Stalin died, the JS-3 changed name to "T-10." Tereshev(sp) did the main work on that series. The US Army "T" series tanks were tests which would eventually become the monster M-103 super heavy tank (120mm gun/crew of six).