Update from the lovely Andy B at SJS:
It’s always nice to hear that we have fans out there but I’m sorry to
have to tell you that I’m unable to confirm or deny that it’s a Thorn.
Back in the late 1980’s and early 1990’s, we worked closely with
Argos, getting individual frames made for our growing number of
customers. This looks like it could have been one of those frames.
Back then such a frame would have been made from a mix of various
Reynolds 531 tubes – different gauge tubes would have been chosen for
each customer. Later we had Argos oversee a batch of Italian made 531
light touring frames – it may have been one of these – or there again
it could be a later copy by someone else.
I don’t recognise the format of the serial numbers but we never kept a
record in any case. Our opinion at the time was that, if the bike was
insured and the owner took sensible precautions against it being
stolen, then if it was stolen we never wanted it to be returned to the
owner, as who knows what may have happened to it during the time it
Aside from the hilarious anecdote about serial numbers, my takeaway from this is that it's 100% definitely one of Thorn's Italian built, 531 tubed, lightweight touring frames. Or at least that's what I'll tell myself :)