David,
It appears that there were two different canvas covers. One for units without the frame guard, and another for units with the frame guard.
There are different part number formats evident in the parts lists. It would appear that during most of the development of these sets the part number format was xx-xx with up to 3 digits left of the dash, and up to 4 digits right of the dash. The later development (which also appears to be the militarization of the design) uses a 6 digit part number format.
With both canvas covers having 6 digit part numbers, I suspect that they were developed for military contracts. The parts list shows canvas covers applicable to all CHI-395, and CHI-396 models ( though not for-397 (office lorry) type).
I presume that a canvas cover was supplied with all military contract units.
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