Gotcha, all great information! You're correct, gearbox full of oil behind the fan on mine. I actually have an original parts/repair manual for my 520, too. I make a point to find original parts manuals for my implements if possible, makes sourcing parts much easier!
I read somewhere that the three blade 2nd stage fan wasn't the best at removing all the snow the auger feeds it. From looking at the parts diagram and the materials it's made from, if I find it to struggle it will be easy to fabricate a new blower fan for it. I'll decide after I get some use out of it! Thanks for the informative post, Dave!