Registered Forum User
GTL
Bikes: ST4s ABS
Posts: 480
Join Date: Aug 2003
Location: Tayside, Scotland
The H1 and H3 bases are quite different. The H1 is round with a flat on one side whereas the H3 is round with a notch out of it. It may be possible to fit an H1 into an H3 fitment but I think the filament position and orientation are also different so it wouldn't focus properly.