Hallo Hendrik,
Not sure if the number is the same, but S/M 6438 is a British contract for Ford Model Number C291WQ. S.M. or S/M stands for Supply Mechanical.
Canadian contracts were "CD" and/or "LV" denoting Contract Demand LV, where LV is thought to stand for Land Vehicles (but this is speculative).
My F15A was built under "CD 1513".
See David Hayward's listing here:
Known British Ford S/M Contracts
Hope this helps
Groeten
Hanno