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If CB it is certainly late enough to be from the Holdsworthy period.

Lugset appears to be Prugnat 62/S, crown Vagner PC.

Shell appears to be Gargatte (Raccords Gargatte Freres).

You mention the BB swiss threaded. This would be more consistent with a cycle from France or Switzerland. Seat stay treatment consistent with a French product. One easy check would be to measure the diameter of the main frame tubes to see if they are BSC or metric. You mention steerer BSC. If frame tubes metric diameter it would tell us fork a replacement. Holdsworthy did employ a lot of Prugnat lugs however.

Have you been able to find any serial markings?

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