is there supposed to be a round gasket between the distributor base and engine block on a 235

..NO..

I think the same answers applies to all GM engines not only the 235.
Tony

All 6 Cyl. Chevrolet engines up to1962 used no gasket because the distributor must be free to turn in the block due to the design of the vacuum advance.
The later 6 Cyl. and V-8 engines use a gasket because the distributor plate rotates rather than the distributor to advance/retard the timing.......like 99% of other engines do.