Now I'm confused ! I do have Orchard & Madden and they are my only source for the non-Norton stuff. The Ariel number looks like a real oddity, doesn't it ? I have been counting the Norton census numbers begining at C3910768 as seven digits.
I think that the contract date of 16/3/1939 is a bit early for C.5109. Twenty years or so ago, there was a query in "The Classic Motorcycle" relating to this contract and a date of 16/9/39 was given. I also have a list compiled by the former VMCC Military Motorcycle specialist, made prior to Orchard & Madden's book and it suggests that production began on 7/10/1939 which is substantiated by the surviving factory records which show the last machine from the contract prior being despatched on that date. Unfortunately, this is more or less the only despatch book from the WD contracts which has survived.
Of course, none of this is of critical importance but I can't help wondering if I have a 1939 or 1940 bike !
Rich