There is no conspiracy. Service manuals are proprietary material intended for service centers to order replacement parts directly from the manufacturer. For that reason, they list part numbers consistent with their own internal inventory control system. Listing length, thickness, etc. suggests that any belt meeting those criteria would work properly, and they don't always do. Was the belt made in China? USA? Is the material and thus, characteristics the same? The fact that some old belts turn crispy like potato chips and some turn to goo suggests they aren't all made of the same material and therefore the characteristics aren't all 100% the same. I have seen some decks are extra finicky even if the same size belt was used.
Now, 40 years later, all of these parts are obsolete and no longer available from the manufacturer so the sizing data is really useful for aftermarket use, but not really for a manufacturer since again, providing such sizes would suggest that the manufacturer condones similar sized belts as equivalent performing and they can't because they simply can't know for certain in every application that aftermarket belts will perform the same as OEM. Some components, for safety reasons, must be ordered directly from the manufacturer or they will list the special characteristics. Examples are transformers, capacitors, etc. Although today, you can order generic aftermarkets to replace a blown transformer, the manufacturer would never officially condone their use since they aren't tested to their specifications. Plus the fact that everything is out of warranty by now, it is moot since there's no longer any reason to take an official position on any replacement part -- probably they'll tell you to buy a new more modern one instead and retire the old out of warranty set.
But if you think back 40 years, a unit brought in for service under warranty would receive genuine manufacturer supplied parts. Otherwise, no manufacturer would pay for warranty repairs unless the factory part was ordered. Furthermore, if you were a service center, you charged the customer whatever the parts cost along with the service charge. Why would you want to risk a call back to save the customer a few bucks? After all, you might save them a few bucks using aftermarket stuff but YOU, as the service center would have to eat any redos. I remember decades ago purchasing some used decks and the sellers even provided me with either new OEM spares or the packaging of the replacement parts used to repair the units. They were manufacturer parts.
On the other hand, generic replacement parts where manufacturers don't typically provide their own such as capacitors, resistors, etc, will generally list the details such as resistance, tolerance, wattage rating, etc in the remarks column. With respect to parts are proprietary such as in many JVC chips, they probably don't since there is no generic equivalent. An example would be VUC0002-001 in the M90. It's merely listed with part number and no remarks at all.
