I always though that the caps were common on the 30 series and TSR and later 22-2 X series recorders , seems i was wrong, I wanted to put a 22-2 cap on my 38 but it jams the roller seems the internal spigot is bigger on the 38 and the TSR 8.
Here a photo says a 1000 words have a look at the right unit (from a TSR 8) and you can see, of course the TSR8 has more straight sides but the 38 has same front as one on the left (that does not fit) i am not sure if the left caps will fit the 32-34 series