Those numbers are ones for service in Canada. The overseas DND numbers have nothing in common. The 42 is the year they were issued. As far as I can see the DND numbers were broken up in blocks which were assigned to various types of vehicles. The prefix (CT, CZ, CL,etc) refers to the type of vehicle and the number is unique to that vehicle. For example, CZ4206605 is a C15 truck and there wouldn't be a CT4206605 or CL4206605. I'm not sure whether the British Z and T numbers ran in the same series or not.
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1940 Cab 11 C8 Wireless with 1A2 box & 11 set
1940 Cab 11 C8 cab and chassis
1940 Cab 11 C15 with 2A1 & Motley mount & Lewis gun
1940 Cab 11 F15A w/ Chev rear ends
1941 Cab 12 F15A
1942-44 Cab 13 F15A x 5
1942 cab 13 F15A with 2B1 box
1943 cab 13 F15A with 2H1 box
1943 Cab 13 C8A HUP
1944 Cab 13 C15A with 2C1 box
1943 Cletrac M2 High Speed Tractor
MkII Bren gun carrier chassis x 2
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