I can't find my post about it either, but I have 2 of them, one working on its original dynamotor PS, and intended for the B17 'Lucky Thirteen', the other I will eventually get going on just a DC supply.  I was thinking that the 522 was featured in CQ's surplus conversion manual as a rig folks converted to FM, and maybe confused myself with the AM/FM issue from that,  if I can find my manual I will check that again.  Details of what the capability of the Germans were comm wise, and why the US played catch up can be read in the post WW II book, volume 1, 'The Emergency'.  SCR522's were retrofitted in B17's to allow them to communicate with British aircraft and ground directly.  Tankers conshawed them from depots and put them in Shermans to do the same thing, and eliminate the step of having to request air support and to direct fire without having to relay back wards, then over to the Brits.  US Comm was pretty fouled up prior to and at the start of WW II, but we figured it out.  The Army had to learn how to properly relay a message from station to station with zero defects  by using ARRL information  after seeing  hams doing it every day.

Charlie in NC