I bought an old Gibson mandolin-banjo the other day. I think it's the MB-JR model, seeing as the hoop diameter is only 9", but it has no model designation anywhere on it. I paid $500, which seems to be the going rate (from what little previous sales information I could find on eBay, craigslist, and auction-house sites) for similar instruments in the low-end range of the Gibson MB line. It was in nearly immaculate condition, so I'm not too bummed about not getting a better deal.
In any event, I'm trying to pin down exactly what model / year / etc the instrument is. Like I said, there is no model number on the gold oval label inside the hoop; all that says is "The Gibson Mandolin, Banjo, and Guitar Company of Kalamazoo Michigan" or something to that effect. There is a serial number: 8023-15, but I can't seem to track that down anywhere online either, although some sources identify 8XXX serial numbers in general for Gibson banjos as being of 1925 build.
9" hoop, smaller than what is quoted for the numbered MB-# models I've seen around.
"Snakehead" style head, with very little decoration, but coming to a nice point.
No binding, no inlays (except dots), no flange, and an open back for the head.
Whatever else you see in the [small, from the craigslist listing] attached pics.