Bear in mind that in the earlier days, it wasn't 'Citizens Military Forces' but 'Citizens Forces'.

I too used to think it was for 'Citizens Military Forces' however the abbreviations under the instructions for Ordnance and subsequent stamping states that CMF is for 'Commonwealth Military Forces' and how to stamp the military district number under the C.M.F.

For some reason i cannot attach the images from the Military orders but will provide them in a message if people ask.