Back to the 7's on the rear boss and 1's and 2's in case matching number. This information came directly from an old guy who worked at HD from 39 to 41 and again from from 45 to 51. In the late 40's it was his job to stamp some of these numbers and this was the procedure that was used.
The case matching numbers were put on the motor cases by the machinist that made them a pair. The number was taken from a work order. Then the cases were shelved until they were built in to a motor. It is possible that they were shelved wrong leading to a few confused cases.
The year and model information were put on the motor by the engine builder who of course would know whether or not it was 61 or 74 and high or standard compression. The motor was then shelved until it was assigned a chassis.
The sequence part of the number was stamped on the motor when it left the engine shed destined for a bike. The number was taken off of a work and job order.
This procedure explains why it is not uncommon to see different numerals between the year digits and the sequence digits and why they may have different impressions. They were stamped by a different person using a different stamp set and the bottom numbers were stamped using a third person and a third stamp set.
In all my years of owning knuckle and pans and I guarantee there are few people who have owned more original motors and bikes. I have only seen the 7's (41 and up) on the rear boss wrong once and the 1's and 2's on the matching number wrong 3 times. It is a pretty reliable indicator of what the motor started as. I also make a special point to check every bike/motor that comes around for serial number/case numbers and look for the 7 or lack of it.
Sorry, didn't really to preach.