It may depend on which year you are depicting, "Hodges & Taylor" has 31 on black for an Infantry Division Provost Company In April 1940 and 79 on black for 1944, but it doesn't say when it changed.
As Rob says, date / location is important. There will have been possibly four different series - BEF, Home Forces, Middle East and NW Europe...What period are you thinking of ?
It would be NWE Europe, June 1944. Basically the invasion time. I know I have seen 79 before. And my pictures now make all the sense! Thank you so ver much guys! I guess the second question to follow would be, was the size the same on the front fender as the size of the markings painted on jeeps or did it change?
For Home Forces, the use of '79' on black for infantry division provost units was confirmed in a GHQ letter dated June 1941. Middle East is likely to have remained different for some time but it seems to have been standard for N.W. Europe.
The photograph that you've used shows Canadian troops but they followed (officially at least) the same scheme.
The 'Arm of Service' serial should have been displayed on each side of the fuel tank rather than mudguard. Sizes for motorcycles were smaller than other vehicles (but Jeeps sometimes seem to have had smaller markings also).
Does anyone care to guess what the writing on the blackout mask is? Interesting photo. They have white webbing belts on so that figures for provost & I like all the gear tied to the back of the center bike. I believe the rolled up thing is a tent. Also shows evidence the they really did carry a water bottle stowed like that. I thought it was just another modern day interpretation. Nice pic.
I don't need to guess, Rick - it says '40 MPH' - this was the standard military speed limit but presumably being Provost on traffic control, they wanted to make sure that they didn't break the rules themselves !