Just saw this ... sorry it's kinda old. I have a reprint of a 1936 Abercrombie and Fitch catalog; they have a page on Griffin & Howe "sight mountings". The model F403 covers most of the double lever mounts, which are either 22mm or 26mm diameter. They cost $32.50 if they don't have a micrometer wind gage fitted ($35 for the F405 for 22mm, or $37.50 for the F406 for 26mm, if they do have that feature). If your mount doesn't have a micrometer wind gage, it's almost certainly an F403.
There's a single lever mount, the F402, which doesn't have the fancier options and come only for 22mm tubes.
I hope this helps!