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I personally believe that the ring & pinion were not lapped prior to installation at the Ford factory. First I don't think Henry would have wanted to waste the time when they will be lapped in after about 500 miles. Second, I have taken hundreds of original R&P's out of Model A rear axle assemblies over the years and have found one thing in common. All have a serial number stamped on the end of the pinion gear. When you clean up the ring gear and hold it at an angle to the light you will see same the same number written with an electric etching pencil on the flat side. The numbers are large and written by hand.
My theory is that a worker pulled a pinion gear from one stock bin, and the ring gear from another and then wrote the number on the ring gear before he installed it in a rear axle assembly.
I have heard people state that they have successfully mated up an R&P that came out of different rear axle assemblies. I would think after a few thousand miles it may lap in. I for one, however, would not do it because it could be initially noisy and result in customer complaints.
Tom Endy