Hi All, my Lucas pages from the 1950 catalog show a DU42R as the correct headlamp, but some feel it was the DU142.
The military models changed to the DU42 around 1941 - Not much point in having a big headlamp in the blackout and the 8" is quite vulnerable to tumbles.
I'd suspect that Norton and or Lucas's stock in late 1945 was mainly of DU42s and they used them up first rather than re-tooling for the larger unit.
The '46 models had the 'for one year only' girders with double headlamp mounts. It ought to be able to measure how wide apart they should be but period photos seem to show that it was the smaller lamp.
Thank you Richard, that makes good sense!