Going through my catalogues, as far as I can make out, no pre-war Scotts were fitted with triangular toolboxes. These boxes were only fitted to post-war Scotts.
In the ’30s, two types of boxes were fitted up to 1934 depending on the model. Leather or fabric fronted rectangular boxes and what I can only describe as triangular boxes having a curved top, about the radius of the mudguard, and having a flat bottom with about a third of the triangle cut off at the bottom. These were leather fronted and continued up to 1939.
If you send me your email address, I’ll forward a drawing of the latter ones with dimensions.