c8 british or canadian markings

[aj.lec]

Can any one tell me what these door markings mean ?
Cliff suspects they may be british or canadian
As far as i can see it says P/4/BOM over a black diamond

Mike suggested earlier quote-

The diamond may indicate a armoured unit ..my first C8, a cab 12 , from 1981 , had the same diamond on it ..plus the 1st Aust. Armoured Div sign on the cowl. Z4544 was the reg. prefix too