The tractors on our farm were a Farmall "H", and a Farmall "C". I used the "C" for any jobs it could handle, since it was nearly the same power, and handled much better.
I left the farm into go in the Army. Years later, after spending a couple hitches there, as soon as I got home from overseas, I took a job of parts manager with the local IH/New Holland dealership.
The guy who had the job before me had ordered some of those canvas half cabs for Farmall "M" tractors, such as you described, for sale in our area. We don't use propane on tractors here, but he must have got his part number wrong..because the dozen half cabs he ordered, came through with the cut outs for propane.
Well, they would work OK on the gas or diesel "M", no problem, but we had to sell them at a reduced price. reduced. So I placed on a high rack with the windshield on top and the canvas stringing to the floor. Fresh out of the army, I was amazed how it looked like a military protective mask, so trying for a bit of humor, I put a sign on it saying... "ELEPHANT GAS MASK AT REDUCED PRICE, WILL WORK FOR TRACTOR CAB"..
Most people got a chuckle over...but then I had one old farmer come in and looked it over very seriously, while rubbing his chin.. Presently he spoke up, saying.."you know, iI think that will work for a cab!".. I had to hide my grin as I leafed through the .parts files..