Re:
We do have another Stuart in the Database. This has a 6D number 037473, do you think this is the same firm?
I doubt that the 'Stuart' name is anything more than coincidence.
I'm also a little embarassed by some of these earlier threads in which I over-confidently asserted my frame (and thence others) as being 'mid-'80s'. Thanks to your frame number threads, dwscrimshaw, I now accept mine as being from 1981-2.
More 'speculation':
It wouldn't surprise me if the more or less freelance framebuilders were simply left to work to their own numbering system from one end of their career to the other, unless circumstances obliged them to conform to an extant system. Thus Tommy Quick taking over from Reg Collard was presumably at some point left with Reg's last number, and carried on from there after his own fashion: What had been a 'first two digits=year' system became a purely sequential five digit system. I've no idea as to what Tommy's 'productivity' was, but as I've pointed out, 150 frames per year would lead to the 69*** numbers running out in 1975, coinciding with the change to Roy Thame badging. I don't think it is fanciful to suggest that '69***' numbering, as the product of one artisan framebuilder*, might not even have reached 69600 by 1975? (I am thinking of Shaun's frame here, and the LFGSS Roy Thame linked upthread)
Other 'outside' builders who might be sporadically commissioned to build some frames when back-orders threatened to pile up would likewise stamp the frame according to their own numbering systems unless specifically asked to do otherwise.
This is just my deduction based on the available evidence- or as much of it as I am aware of. That doesn't make it 'true', of course..
*An 'artisan framebuilder' being a person who single-handedly builds a frame from one end of the process to the other- tube-mitreing, lug-cutting, and brazing, or at least supervises and quality-controls all those stages that might be performed by apprentices.