I have given this some serious thought and have consulted the Aermotor god. We have come up with the following explanation for the 502 that Roger has:Given information:
1. The wheel guide has a 1929 date
2. The hub is the kind with the holes for the U-bolts (post 1925)
3. The Yoke has been updated to one with the Oiler Ring (is there a hole in the wheel guide where an oil flipper would have been installed?)
4. There is an 1920 patent date (which is the patent for the oiler ring)
5. The stock market crashed in 1929; the depression ensued.
6. People did not have money like they once did and if their windmill wore out and they needed a new one, they would not buy anything they didn't need (to upgrade from a 502 to a 602 you would need a new tail bone, buffer device, and tail vane)
7. Aermotor hubs and shafts from the original to the 602 will mount on the same sized shaft (diameter) but varying lengths.
8. Aermotor has a marketing history of selling prior models with more modern upgrades (later frames with earlier wheels: ie an RA frame with a bell shaped hub) to satisfy the customer long after the production run has ended and as long as the products are offered.
9. Aermotor was in business to be profitable and in order to be viable during the depression, they did what was necessary to make a sale (like making a customer a frame and selling it so the customer did not have to upgrade all the other parts)
10. The single stroke pitman arm has a 500 series number BUT it is in an "I-beam" style (like a late 602 and 702) BUT it mounts to a big gear with pegs and not bosses like the later gears have.My "logical" conclusion:
With all those assumptions, I think that the customer that ordered this mill, ordered a 502 so he did not have to also purchase a buffer device, tail bone, and tail vane. I believe that the date on the wheel guide is correct. I believe that the frame is a special run, maybe even a second special run (hence the 2 dots after the model number) for customers that needed that model frame to fix their water system at their home/farm. I believe that the yoke with oiler ring was original to this mill. I am going to go out on a limb and say that the bottom of Roger's 502 is probably cupped to accept a modern mast pipe (like on a 702 or late 602). I believe that this 502 is an example of Aermotor's dedication to providing what the customer would buy in a time when money was tight.
Now I can sleep better....