The Centurion tanks fitted with 20mm co-ax should have been a model of the British Polsten cannon if memory serves me right. The Curtiss SB2C had four M3 cannon (sometimes written AN/M3). This was the penultimate variant of the US built variants of the Hispano HS404.
Er, isn't that 20mm Polsten on the Centurion only used as ranging gun? If so, it shouldn't appear in the OOB due to limited ammo and no combat use, I guess.