Thanks once again Ivor for some wonderful pictures. Pre-war bikes and Service Dress - Old photos don't get much better for me.
I think that RMT 540 is actually a 1939 C10 BSA. If the census number begins C3995, that would confirm things. Delivery of contract C.3939 (C399327 - C399926) commenced in May 1939. What rank did Callow hold in the photo ?
The Enfield is also a 250, a WD/D from contract C4452 (C3922045 - C3922944)
Both of these 250 sidevalvers were regarded more as light communications machines and were generally retained for home service.
I suspect that the 'other' desert Norton is a slightly later one from Contract C.7353 (C4386027 - C4403026) Machines up to about C4392000 still had those rather peculiar double steel toolboxes visible in the picture. I now know that they were 'Footrests, trick riding, officers for the use of' which also explains why most of them are so dented and twisted !
Rich.