u/MLGOV

Hello all I got this old riverside arms 28” 12g single shot last year from flea market, I was wondering if anyone know anything about these what shot size I can use up to or any info on it would be appreciated!
I know that it can only use 2 and 3/4 shell and lead shot and possibly made pre WWII , I just wanted to know what the limit for shot size if anyone knows!