Nicholson's Shop in Dundee still exists (see http://www.nicholsons-cycles.co.uk ), and were once an agent for David Rattray's Flying Scot's
I'm not sure however that they ever built any frames "in-house" - A lot of Scottish shops in the 70's & 80's sold bought in or imported frames with their own branding to try and attract the local racing clientele, but these were generally no match for the imported named brands they were trying to compete with... Plenty of Columbus Aelle, dodgy Italian paint jobs and rumours of being built by Italian ex-con's....And the 'shop' frames were not much better :-0
Your best bet is to e-mail Nicholson's via the website - The son of the original owner still works there, and his father I believe is still alive but retired
Alternatively I'm sure if you ask nicely Bruce Robbins will disconnect himself from the mains long enough to speak to Vic - the local "Dundee" builders expert....
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Bob Reid
Stonehaven
Scotland
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The Flying Scot Website - http://www.flying-scot.co.uk/