Wetdog,
What is the engine number? This should be stamped on the drive side, behind the flywheel. It appears to be a WA6, Mk II, which would make it 1923 or later
My 500 Barr and Stroud is said to be a 1923 build. The full history of it is known since it was delivered (here in Oz) in 1924, so there is a small measure of credibility to that.
My engine number is 231324. It may be a happy coincidence that it begins with "23", but is it number 1324 of 1923 or number 2313 of 1924? More engine numbers will help.
cheers,
JFerg