How Do I Learn Sheaffer Nib Size

I'm a newbie, so be kind please. LOL

I was given a 10 or 15 year old Sheaffer Connaisseur. When I look at the nib, I see 18K and "II50" or something like that. But nowhere do I see a F or M. How do I learn what nib size it is?

It writes a lot like a Safari with a F nib.

Thanks

I had to take out my one Conni to check. I don't believe there is any marking on the nib itself and nothing stamped on the underside of the feed (which sometimes exists on other Sheaffer pens). If it is simiar to the Lamy F, then it probably is an F as both company nibs are a bit on the wide side; it would certainly be wider than a Japanese F nib.