Is there a standard for pool return fittings?

My Sylvan-built pool (ca 1990) has two returns in the shallow end which are simply PVC pipe sawn off flush with the plaster. When winterizing, I plug them up with #4 rubber plugs. (There is another, separate return for the pool cleaner.)

I have seen other pools where the return is terminated with a threaded fitting, which permits a diverter to be attached (during pool season) and (I'm thinking) might permit the use of a threaded plug for winterizing.

My question - is there some standard type of fitting now in use? If so, what are its dimensions? When I do some pool renovation this summer, could I attach such a fitting to the existing PVC? Thanks.