Yep, some engines don't have a clip on the exhaust side.
To prevent the piston pin to come out, the piston pin hole in the piston is a bit smaller there......
But for balans reasons, most engines do have clips on both sides.

Some years ago, I've had an OS engine with no circlips at all!
The liner was rotated about 45 degrees, so the piston clip moved just between the liner ports......