STOKED to hear you offer Gottlieb 3/8 now! Was literally going email y'all asking if/when you might start carrying these after I was done browsing pinside this evening. Good stuff.

3/8 is for most Gottlieb flippers (3/8 width).
Mini flippers would be like on TAF, the flipper on the left above the in/out lanes.
There's also the beaded flipper which the rubber is flat on the outside and has a bead all the way around the inside that fits into a groove on the flipper (many Gottlieb mini flippers are beaded). IDK, maybe this is what your mini flipper needs? Unless you are saying the rubber you had on it was round like a regular ring.