US 20mm cartridges - T7?


I am currently trying to pin down the 20mm automatic cannon cartridges being tested by the US in 1945-50, specifically those for the ARF T33 and T74 guns, but am becoming rather confused by Chinn (not for the first time!).

I understand that the T74 cannon was just the US designation for the Mauser MG 213C revolver gun, of which the US had acquired an example in 20mm calibre - therefore chambered for the 20 x 135 Mauser cartridge.

I also understand that the T33, T33E1 and E2 were chambered for that giant 20 x 158RB cartridge, which had the case designation T5. This (I think) fired a 2,000 grain projectile at 3,500-3,775 fps.

But Chinn then says that the T33E3 was chambered in a cartridge with a smaller T7 case which fired a 1,600 grain T61E1 projectile, also at 3,775 fps, and which had a 900 grain propellant load. This round was supposedly designed originally for the T74, but I can’t imagine that they were intending the adopt the Mauser 20 x 135 case…

Can anyone please tell me anything about the T7 cartridge case, especially its dimensions?