It could be anything to be honest! The original USA reg. number would be 40xxxxx o that does not fit. A British Army number would be Zxxxxxxx. We did not use many Whites, most of the British halftracks were IHC. I did find a contract for two M2s, Z4857971 and Z4857972.
If it were a unit number, it would be more likely on the rear bumpers and in conjunction with symbols to indicate unit size. If it is in a single row on black rectangle, it's more likely a French Army registration number.