My observation of WD serial numbers suggests that the small headlmaps were fitted to machines delivered under Contract C4631 which RAOC records date to 13/9/39 and a dated photograph from RASC Feltham shows them being checked there in December 1939.
The contract was only for 300 machines and I haven't found any reference in the Ministry of Supply records, but early September 1939 is around the time that Norton were putting Model 18 engines in WD16H frames and BSA were turning out M20 framed Gold Stars...A few Triumphs with smaller civilian economy headlamps would not have been seen as a problem !
So it would be appropriate for a bike from either of these contracts to have an 8” headlight
The smaller headlights associated with the 3SE models impressed in the early stages of the war
Job
For the two contracts that you've mentioned, I'd say definitely 8". Looking at photos again though, it seems that the early part of C5108 might well have had the smaller lamps too. This was also a late 1939 contract.
It seems that by 1940 they had standardised on the larger headlamp.
