If you have access to the car, there are a couple things that would indicate 'factory 100M' provenance, and if they're there then getting the BMIHT inquiry would be worthwhile. Bear in mind that getting the cert can take a while, and if it's a real M the car may not be for sale long.
Specifically, look at/for:
1) the distributor. The M used a special distributor--there are two part nos. that I don't know offhand--and this would be difficult to fake
2) the M had special carbs; not just any old H6's. They have hand-scripted markings that, again, would be difficult to fake
3) the original louvred bonnet is different than the aftermarkets but, sorry, I don't know the details
4) the special bent shroud bracket that is used to clear the cold air box is not sold by any suppliers that I know of, though it would be fairly easy to make one with a metal brake if you had the dimensions
5) I think the sway bar for the Ms is thicker than a regular 100s, but that would be easy to fake
Aside from those, it's relatively easy to build up a car that looks like an 'M' (anybody can bend the front radiator support). Perhaps Bill can supply the distributor and carburettor info; if not the concours guys know them.