My guess is that it is J. GAHM. I suppose it could be a mis-print. I don't think that they are that rare but they turn up in some spectacular colors. It's still a neat old bottle (even if it is a Gahm).
J Gahm & Sons was an authorized Schlitz distributor. The bottle most likely had a Schlitz label on it at one time. There are versions that are embossed J Gahm & Sons on one side and Jos. Schlitz on the other. As Blobbottlebob metioned they came in some nice colors.