So far we do have 3 digit number in the early 70's 4D system, but the custom frames, Harding, DBS, Pollard & Stuart start the other side of 1800. We don't have many so it is possible this is part of that group, but would be early 70's.
Charlie Roberts certainly has a history at Holdsworth's and interestingly in 50's Holdsworth factory serials were placed on a rear dropout - something I often wonder if Roberts' doing as I believe he was foreman around that time.
However Kman's beautiful Roberts looks like has own serial method. He apparently had a passion for time trialing and shows in his builds. This frame uses his own simple sequential system (didn't go very high) and is likely from early 70's. When he built for the trade apparently he would use that particular brand's system and placement - the Putney shop being an example.
There is a bit of a serial registry on Classic Rendezvous that possibly people have seen. Apparently after he died in late 70's his son Chas persisted with same system until early 80's when first few digits were used to signify date of build. Torqueless and as he says Roadking has a lot more knowledge of Roberts.
http://www.classicrendezvous.com/Britis ... gistry.htm