looks like a hub is missing,
I got a few wheels off a old MM plow, I look at tomorrow, (mine was newer than yours, I think), but one could make a hub, from some DOM tube, and a plate cut in a circle with some holes to bolt the wheel on, drill a hole in it and tap it for a grease zerks, some washere one the end so it does not wear out the cotter pin that holds it on, the factory one usually had a cap on them that went over the end and the cotter key, the hub was long enough to fit up under the cap washer, so when dirt was pulled up around the wheel, it would not fall in the cast bearing.
most likely it similar to what the colter hubs are like if your plow still has the colters, on it,