Jason, the Chilwell lists do not give frame numbers linked to contract data for Canadian contracts, only number blocks are listed, however using John Farley's 'The Standard War Time jeep vol 2' it would appear that the contract you quote above would be for 1000 units in July 42 going to an ongoing Canadian contract.
The production for July was MB frame numbers MB155101 to MB156497 =1396 units. The other 394 units going to an ongoing British contract SM2428 serials M4923602 to M4923996 =394 units (notice there is a discrepancy of 2 units here).
Farley lists two known Canadian serial numbers from this July batch namely CM4231488 and CM4231785. The Chilwell list only gives the full allocation of 42##### number 4200000 to 4299999 this will cover all types of trucks so not much help to you I'm afraid.
However if August 42 is looked at then contract 505 was still ongoing and two serials from that group of 700 units supplied are CM4232453 and CM4242123.
Therefore it would be reasonable that your MB would be around the CM423#### range
Pete