I have now heard back from the Vintage Motor Cycle Club (VMCC) who hold the Triumph's works record in their archives. I had to send them photographs of the bike as it is now, with close ups of the engine and frame number.
On the telephone they informed me this is because they were receiving requests for details of engine and frame numbers for certain marques - and then noticing that 'lo and behold' a suspect looking bike came for sale shortly afterwards with those numbers.
The classic example of this is BSA's rocket gold star - with that model commanding the kudos and a price to match (but many parts in common with another model), fakes are rife. The VMCC estimate their are twice as many RGS's registered than BSA ever produced ...
Anyhow ! I didn't get a lot of information - but what is there is good. The bike was dispatched from the factory on the 10th of July 1960, to Burns of Stafford. The order number was 4461, and the invoice number was 35345.
The parts book tells me Triumph supplied two types of rear wheel - a standard hub or a QD hub, QD being quickly detachable. Our bike came with a QD hub - so I'll shortly be looking to see what we have in the garage....
The gearbox ratios were for a solo (as opposed to a combination / sidecar) and should be numbered 116774 (though that last digit could be a 6). Looking at our box that number is different:
So, looks to be a bit earlier ? The front though, with its little chrome cover and two screws, point to it being a slick shift box (that disengages the clutch on activation) .... so a bit more digging to do here. When did the slick shift come in etc ? it may be that an older outer casing was used to replace one damaged by a chain break ?
So a bit of history in place and a few lines of enquiry open to pursue.