I based my assumption of a lighter, more highly-processed tubing on the fact that the 103-tubed bikes no longer used any spot-welded cable stops, but switched to more-costly brazed pieces. This suggests that there was a necessity for this, that the newer tubing perhaps didn't endure their fatigue-strength tests after spot-welding.

The 103 "Carbolite" designation was the first change in labeling in perhaps decades, so I think that it's likely that Peugeot after so many years actually made some improvements to their in-house manufactured tubes. A frame-weight comparison would be the tell.

The bigger news with the new 1979 frames was their improved sportiness. The first-year 103-tube U09 model actually bore a "Super Sport" inscription on the top tube and the bike performed quite well (much more responsive to pedaling input than the older frames, I have both), especially after the rims were replaced with alloy parts (the U0-10 model came with alloy rims).