Although 99th Field went to France with 48th Div., they were swapped with 18th Field of 2nd Infantry Division in February 1940. This was part of a GHQ plan to move a number of regular units to Territorial divisions in order to bring them some experience and of course those territorials moved to the regular divisions also benefited.
The BEF GHQ markings lists of March and April 1940 confirm that 99th Field Regiment displayed 2nd Division's 'Crossed Keys' formation sign and the Arm of Service serial '5' on red over blue.
Putting all this into practice on a motorcycle is less straightforward as space is restricted and there seem to have been considerable variations in the interpretation of instructions and thus how the markings were applied. One unit might consider the leading edge of the fuel tank to be 'forward facing' and another might try to apply a small sign to the tip of the mudguard which can mean that what fitted on a Norton might not on a BSA etc.
Some time ago, I put together a thread on the WW2talk forum in an attempt to show some of the variations.
http://ww2talk.com/index.php?threads/bef-motorcycles-formation-signs-arm-of-service-markings-and-census-numbers.34843/page-2
You'll see that there is a rather poor image of an abandoned M20 with the 2nd Division sign on the left hand side of the large early-type fuel tank and also a Norton with the sign on the right. It's impossible to tell if they had the sign on both sides. I suspect that the BSA did, with its census number on the front number plate blade but that the earlier Norton didn't.
Choosing where to put the Arm of Service serial (5) on the RA red / blue square is difficult on anything other than a museum exhibit as the only consistent location was the rear numberplate and of course we're forced to put a registration plate there. You could try the tip of the front guard but there isn't much space and I have no provenance for a Royal Artillery marking there.
This Morris-Commercial 8cwt belonged to 99th Field Regiment.
On the petrol tank a 5" square carried the unit number...The div. sign (on the tank) was of similar overall dimensions though obviously not necessarily square....
Sign writers used a set of 'default' colours in the absence of specific instructions...The signwriter I use has those default colours and calls them 'signwriters blue' 'signwriters red' etc...
These colours look very like what was used in the R.A. examples shown in colour that I've seen...So you need to find a traditional signwriter...
Here's the cobalt blue of the Engineers...not the default colour as this shade was particular to that branch of the services...Ian
email (option): ian@wright52.plus.com