It is fairly simple. The point to the discounts on bulk, non-profit, carrier-rate, Zip, Zip+4, etc., is that the mailers needed an appropriate permit to mail their large mailings which ALSO had to be sorted/bundled/trayed/labeled per specific instructions. The mailer does a significant part of the work for the USPS, they get a discount in return.
I expected every stamp collector to know the rate history cold, going back to at least 1950
Nope. I don't memorize what I can so readily look up.
In the years since the transportation coils were phased out, much stamped mass mailings have a generic stamp applied and the mailer pays the balance of the cost behind the scenes. B&W have termed this a "false franking". It is often impossible to determine exactly what was paid for a given piece of mail.