What a gorgeous bike... but I'm biased, I just love any Ellis Briggs anyway (mainly because it was the first lightweight that I ever raced on back in 1968 - but that's another story).
Anyway, have you thought of just contacting Ellis Briggs direct with the frame number?
I believe they have pretty comprehensive build records there still and as long as they're not suddenly deluged with dozens of anxious E-B owners trying to find provenence for their frames they'll probably be inclined to assist.
They were quite helpful when I phoned them a few years back to try and ID a frame I had bought as an "E-B" and which they told me was no such thing. It turned out to be a Raleigh in the end by the way...
Having said all that I believe we've had this conversation on CR recently - RE: Builders running businesses actually having the time / inclination to search their records for info etc., - So perhaps wait to see if you get any response from the list, and keep contacting E-B as a last resort!?
Maybe there is an Ellis Briggs afficianado on the list who could decode the frame number or knows how their numbering system works?
Ian Briggs
LUTON - UK