Well, here's what I've found . . .
Kit 83843 is fine for modeling the first three or four M1939 vehicles. These vehicles all had side skirts with trapezoidal access hatches, a turret base with vertical end faces, and a two-piece rectangular driver's hatch. Only one of these had the frame aerial. Some number of these had the disk idler instead of the spoked type, but neither Hobby Boss kit has these but I didn't consider this omission to be a deal breaker.
Kit 83844 is fine for the last three or four tanks that featured side skirts with rectangular access hatches, a turret base with sloped end faces, and a one-piece oval driver's hatch. None of these had a frame aerial and I think all might have had disk idlers. This kit has both types of late side skirts but don't use the trapezoidal hatch versions. You can use these with kit 83842 to represent the odd cylindrical hatch tank refitted with this skirt type.
These two kits have nearly all the same sprues, the difference being:
83843 has M1 (frame antenna, vertical turret base ends) and S (night fighting light lenses)
83844 has N (rectangular hatch side skirts, oval driver's hatch, sloped turret base ends)
At least that's what I got from my research, which admittedly did not get into the intricacies of tool stowage and ignored the disk idler omission. What was the combination of features on the 83844 kit that seemed improper, Darren?
KL