The star hub setup was used until e76 with the 58-62 backing plate, drum, brakes and 36-E76 axle. The setup does suck with having to run 2 spares. I also want to run the 76-79 axle, brake drum, backing plate and the mid star 43551-76 sidecar hub. The 43551-76 mid star sidecar hub is different than the regular mid star hub, you have the correct mid star sidecar hub in the picture in your first post. You will have to use the mid star sidecar hub 43551-76 as a spare and run a washer or spacer along with it when used as a spare on the bike. You can measure the difference between the bike mid hub (43540-67 or 43540-76A) and the sidecar hub 43551-76 on the brake drum side.
The only difference between the bike hubs 43540-67 and 43540-67A is that the 43540-67A has 2 holes drilled in the bearing side to hold a wheel bearing lock nut retainer 43592-70 that is used on 70-72 bikes.