Chevy C10 brake booster

[wrangler160]

Hey guys, I have a 1981 Chevy C10 that I am in the process of rebuilding and almost have it driveable with the exception of one thing...brake booster...

I went to the parts store to buy a new booster and they asked if it was a JB5 or JB3 (I didn't know the difference) they told me to check the sticker in the dash and it would tell me. When I checked the sticker it said J50 and no one has been able to tell me anything about J50 brake booster. Has anyone else had this problem or can someone shine some light on this for me? Thanks.