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26.8mm O.D. for top tube may be incorrect

on a metric tube set this tube would be 26.0mm O.D. and on a BSC tube set this tube would be 25.4mm O.D.

if you disassemble HS it shall give opportunity to examine interior of head tube to see if there is evidence of fastener holes being filled in

from down tube and seat tube measurements it sounds like tubeset is BSC

the combination of BSC dimension tubeset, Ateliers de La Rive tubing and the HS is suggestive of a Belgian origin for the frame

perhaps some of our low countries experts shall be able to comment...

fabiofarelli Italuminium JaccoW LucasHartong non-fixie




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addition -

if, for example, frame produced by one of the hundreds of cottage industry framebuilders in flanderland for the private label house brand of a retail cycle shop it may be unlikely to ever determine a specific producer.

OTOH one simple check which could be made is that of shell width. a measurement of seventy mm rather than the sixty-eight mm one might expect for a BSC shell would tend to indicate the maker Flandria. this maker placed long numerals only serials into the left side of the seat lug. here is an example constructed with a BOCAMA lug pattern paired with a MAEDA ends set -



does fork have a flatted (keyed) steerer with keying at front? this is a Flandria trait.
here you can see the steerer top of the same frame as seen in the above image -


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