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correction - referred to the symbol on your bottom bracket cup as a digraph. it is actually a trigraph since it shows the initials of the two brothers, T. & F.
the die used to strike the symbol was not always in good alignment with the work surface and in some cases the T cannot be seen while in others the F is faint or missing.
my error: my first sighting of the symbol was in the Kitching Handbook and the T was not clear. incorrectly interpreted it as a seraph on the W so always thought of the brand/name as WF rather than the correct TWF.
This spindle is discussed in the post by forum member Munny I linked to in the earlier message.
The spindle marking of 70 could refer to intended use with a shell of that dimension.
Keep in mind that gazillions of bicycles were produced by Flandria and other Belgian makers which employed a 70mm BSC shell.
Britain and Japan also manufactured cycles with 71mm shells and BSC threads.
If you would wish to pursue your investigations further you might like to visit/join the francophone Belgian cycling forum veloretrocourse.proboards.com
Poster Munny is the moderator there.
Did a bit of online searching for the Ron Kitching "Everything Cycling" Handbook/Catalogues.
No shortage of mentions but could not locate the downloadable PDF.s which were formerly around.
DisraeliGears offers the 1983 book in a non-downloadable format but that is too late for TWF products.
The edition of 1970 would be a good place to begin for listings of TWF items if you can locate it.
IIRC bottom brackets and headsets are shown but not cranks.