Originally Posted by LWaB
Yes indeed! You guys got it. BRAVO!
Back in the postwar days (and probably earlier), long before racers became pampered overpaid sissy-boys, it was not uncommon for a local racer to have to pedal himself to regional races under his own power. These holders would just bolt on under locknuts or wingnuts of your commuting wheels with heavier tyres, and were offset to prevent the hubs of the spare race wheels from hitting the spokes of your functioning wheel. You'd want to use heavier wheels for the trip to traverse the rutted rural roads. Road races were often ridden with single-speed or even fixed gears, so the spare rear wheel was not very heavy at all. And, it would be fairly simple to switch from a rear Sturmey-Archer gear hub to a single cog wheel with your chain set for a similar cog size.
The examples I've seen in photos show these mounted on bikes always attached to the front wheel. And mounting the wheels just above the hub would probably be like riding with lightly loaded low-rider panniers. The spares would be positioned forward enough so they would not impact against the down tube during sharp turns, and they were of course high enough not to drag on the ground.
Times have certainly changed. I hear that some parts of the UK even have central heating and air conditioning now!