I believe I've seen this particular configuration (two bosses side by side) on the chainstay bridge before. But, why?
Threaded for fenders ?Is it a touring bike, extra W.B. on down tube, any on fork for low -riders ?
Wasn't there once some type of brake caliper that was mounted in that location? I'm thinking it was called a U-brake or somesuch?
I've never seen a fender that used two bosses side-by-side like that. Those look more like rack braze-ons that you'd see on the seatstays.
the u-brakes used something similar to a cantilever brake boss. Maybe it's for a dyno, otherwise I'm at a loss
Woah that's weird. Any other braze-ons on the frame? I see the bottle bosses, but the recessed bridge doesn't scream touring to me.
It's a sport touring frameset. Likely custom built. It features three pairs of bottle bosses and requires nutted calipers. There is an eyelet per dropout, but no other bosses for racks, etc. I'm not sure whether or not they are threaded. The frameset does not belong to me.
It doesn't make sense to me to have two fender mounts there. It could add a little more rigidity, but most fenders have a round profile, so this doesn't jibe.
I would have to assume they are for a fender. If they weren't you would need to cut the fender short to leave room for whatever else would attach to the bosses, and use a clip to fasten the fender to the bridge. Like you said two bosses don't make much sense for a rounded fender but it's probably a weird request from the customer? Custom is custom I guess...
Another thought: maybe it's some kind of kickstand mount?
Did you contact the builder and ask? everyone else has to guess..