Fat backs anyone?

In the 70s M&W made cast iron rims they called "Fat Fronts". These were primarily offered to put fatter tires on the heavy rims on the front of tractors. The sales literature also showed them being used on the rears of combines. Look closely on yours for a M&W logo. 

I have an early M with cast rims is that what those are?

These  cast iron rims use a 11.00x16 tire. The M gleaner does show them in the parts book, but they are a different number than the L. That size of rear tire seems a little big for an M. I think that they are a regular Gleaner wheel and not an M&W?? I can look closer for that. They might indeed fit some tractor front ends ,but the spindle dimensions would have to be the same. They must weigh 200 to 300 pounds apiece???? Clumsy as hell to move around especially with a flat tire.

Yes and no. That is exactly what they look like. Mine have a multi rib tire on them,but any style tire tread would work as long as it's the correct size. They don't use any lug nuts,the wheel is milled for the bearing races, so it fits directly onto the spindle. Those early L and M combines just used a 6 bolt hub as did the G, if they had steel rims.Later production used an 8 bolt hub and I believe heavier spindles too.

The M&W ones would bolt to standard hubs, so they would work on both tractors and combines. I know they were available with the standard 6 bolt AG pattern, they might have also been available with the standard 8 bolt AG pattern.