I have a fixture that I need to replace the bulb in. The old one was shattered and removed prior to me getting it so I don't have one to compare. It takes a 4' tube and the ballast is a T12 ballast, however the socket looks different than T12s I'm used to. Some quick research shows that I need a high output T12 vs a standard T12. Do I have that right? And is there a major difference other than the socket? Or am I completely wrong? I just want to make sure before I do anything.

Yea, is it outdoors or in an unheated space? My only experience is decades old but thats the only place we installed High Output fixtures. They will fire off in freezing weather, the regular ones not so much.

IIRC the sockets are not exactly sealed but are somewhat more weather tight as well.

The fixture itself was designed to be used outdoors. Whether or not ill use it for that has yet to be decided.

Time for LED upgrade

Yes, F48T12 HO is correct if the ballast says 800ma on it.
The High Output lamps and ballast were for reliable starting and operating down to 0 degrees. There is usually a temp rating on the ballast. Often used in outdoor signs or area lighting.