The 66 Assembly manual specifies the 380334-S rivet, same size, different mandrel, and as the question was for a 65, I used the 65 book. When I move my "K" Fastback out, I'll look at my 65 Fastback. I used Orchard Supply Hardware (a local store, Jeff knows) as a source for my 66 Fastback. It was a perfect match to the rivets I drilled out. I don't see where bumpers came into the picture. I got some bumpers somewhere (it's been several years, NPD?) that "fit". Remember, Ford used a new part number if a different supplier was used, so it could be an identical part. My notes show that 359767-S bumper (also used on 66) was replaced by 380546-S, no source reference, no date.
Get a copy of the AMK Products Guide To Ford Fasteners 1955-73 to get dimensions and a picture of 99 pct of the hardware.
As to the Osborn Assembly manuals, 50 percent of something is better than 100 percent of nothing. Mine are marked up reflecting what I found on my Fastbacks (and hardtop) plus what others have documented.
Jim