Re the fillet brazed seat cluster, here’s a comment from a former Bontrager employee:
“Mine is half fillet brazed. In the beginning of building production OR frames, we didn't yet have a full frame tacking fixture. We tacked the front triangles on the fixture and then the rear triangles were added in a separate process with a crude drop out locating fixture that clamped off of the seat tube. I don't remember what the reason was (I wasn't yet involved in welding/brazing processes, just a machine operator becoming a machinist), but Keith decided that we needed to braze the rear triangle on instead of TIG welding it. So it had a TIG'd front and brazed rear triangle. When I get back up to my shop, I'll dig it up and take a picture...”