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I'd guess they would number maybe unless it was built for another company which apparently happened. The KoM model always seems to have used nicer, racier tubing than ST though. There is the Mercian 'classic' that used ST, but that does not have the distinctive heart in the lugs. Confusing 😁
Maybe not Mercian, but another similar maker. Have you tried a bottom bracket yet? Would establish if it's British thread.
Squaredisk
actually yes, it does look a lot like the KOM with the seat stay and lugs and eyelets. Would Mercian still number if it was a custom build?