It's in the same style as the numbering on the AEC's in B n T days, were they numbered in any other place other than the front?
The plate appears to be too small to have been cut from the aluminium at the front of the cab.
I would have expected to see traces of the old green paint, unless they were re-sheeted at some stage?
Looks like a bodge job done to repair something, a primitive means of renumbering or block a hole I imagine, and then fell off? A lot of re-use of old material went on before, a contact of mine found the GSR enamel nameboard of Athlone, but was gutted when it turned out to be just one half of it. It had been cut in two to block a hole up in something. Saw an old BOVRIL sign once that had been made into a backplate for a pot belly stove with a hole cut through it for the flue.