My, I think you have explained the reason my rear fenders are a little "different". I talked to my mother this morning and she said that Dad got the tractor with steel wheels on it in '43...apparently it came from the factory that way. He later traded those for a snowplow. This was about a year before I was born, so I never knew about it. I suspect yours came with steel also if that is the correct reason for the fenders.
My serial numbers are B58812 for the tractor and BE60390G for the engine.