Ok, but this 20 round one from a Swiss Oerlikon is real enough for you to buy it:Mmmm that's wikipedia, not always the most reliable with military tech.
The Type 99 (an oerlikon clone) was mounted with 60 round drums with turret mounted weapons from what I can tell.
To my knowledge there isn't any pictures of smaller drums on the weapon.
Oerlikon 20mm Drum Magazine - 20 rds
20 round drum magazine for the 20mm Oerlikon Anti-Aircraft Cannon. Features a windowed round counter in five round increments.
edelweissarms.com