Hi, the 1930-1940 FB frames which I referred to earlier did not resemble this frame in any way. The fork crown was a plain casting and the lugs were of simple outline and certainly did not have any fancy slotted cut-outs. As a precision engineering company, you can be pretty certain that the top tube as shown on this frame would have been of the correct length and brazed-in correctly. If you visit the "chainring tattoo project" on Google you will find the silhouette of the FB chainring.

I don't think that Dawes-man's FB decal (which BTW looks like it's a modern thin vinyl sticker) has any connection to the Fratelli Brivio, and since all the other transfers look like water-slide decals (for china cups or other houseware decoration), I think it was added recently.
Nice to have the link to that collector's blog, thanks for finding it!

This is how cotter pins install if you don't file the taper to fit!

Hi, FB original frame decals are still available, depicting an eagle on a globe.

Fratelli Brivio varnish-fix decals for their own bicycle frames and possibly for motorcycle petrol tanks are still available in Italy.