While working on my current vintage project I noticed a braze-on under the top tube that I've not seen on any other bike. I don't think it's a number hanger, and suspect it's meant for a strap for a frame pump. Hole is unthreaded and fitted with a plastic plug. Can anyone here confirm what this was intended for?

Cool paint. What is the bike.

I think that is for a removable number tab so you can use the pump peg on training rides

We really need to see more of that bike, please.

The bicycle looks like Giant Allegre, about 1992. I wouldn't expect a number peg at that level. Had it been threaded, I would have suggested a mount for a CO2 cartridge holder, as an alternative to a pump.

Edit. CO2 cartridges were a popular option with triathletes, who often used mid-range sport bicycles, though dedicated triathlon bicycles were starting to make inroads at the top level by this time.

Probably not the OP's. but here are pictures that appear to be the same model.

More located at: https://nilbybike.wordpress.com/2009...giant-allegre/

Per request, here's another picture of this bike, and as T-Mar already noted it's a 93-ish Giant Allegre. All original except for the rear derailer, here's the 'before' picture of the bike as found. I'll be posting more of this in the future when my restomod project using this frame is complete. Also, if anyone knows where to find a Park HTS-1 somewhere in the greater LA region let me know, this has some very slight front end damage - not enough to have a negative effect on the bike's handling, but I'd still like to press out the slight bend anyways:

It looks like the braze on looks a bit too far back from the head tube to be a race number tag holder.
As also mentioned already, the race number tag holders were usually just present on top of the line race bikes and not on mid or lower level models. But, who knows, maybe they added it on to the frame as a feature for entry level racers to go for.
Maybe a period catalogue from the manufacturer can better confirm what that braze on was really for.
I do see a slight fork "bendage" on that bike in the pic. Hopefully it's just the fork and not the frame that's bent as it should be easy enough to repair that fork.

The subject bicycle appears to be equipped with Shimano RX100. The only years it came with RX100 and this style of frame was 1991 and 1992. In 1990 it came with SunTour GPX. In 1993, it came with SunTour SL derailleurs and the frame was updated with an extended seat tube and was only available in blue. I don't have the 1991 catalogue but 1992 came in that colour scheme, which was called Meteorite Melt Red w/Yellow. The serial number should tell us the exact manufacture date. Neither the 1992 or 1993 literature shows the subject boss.