Well after taking some measurements I'm 95+% sure that this is from a 60 series. It's a little over 52" between the heads of the bolts that bolt the brake backing plate to the axle, compared to my 40 which is closer to 48". Question - are the 60 series axles only longer on the long side? Or did both axle lengths change? I haven't been able to pull the long side axle shaft out yet to check it, but it looks to be about 3-4" longer (judging by the housing) than my 40. Makes me wonder if the cut and turned axle is from a 40 or a 60, maybe Timm could take some measurements next time he's at his storage unit?
It being a 60 series would explain the emergency brake and the drum size and configuration, among other things. I will check to see if it has 3.73 or 4.11 gears next time I get a chance.