I was trained in 1977 at the Scubapro factory here in Italy, at Casarza Ligure, where most SP regs are built.
Training was free, as I went there together a friend of me, Marco, who had just started a business as a diving shop, and I was there for becoming the trained technician of the shop.
At the time SP was producing just 2 first stage models (MK2 and MK5) and two second stage models (108 and 109).
The training was most about assembling and tuning NEW regs, in fact after explanations I was sent to the assembly line, and I had to assemble and tune a few new regs which were going to be sold. The only used reg which I serviced on that day was my own MK5+109. The training was not particularly deep, it was the same training they were giving to their workers before sending them to the assembly line. No theory, no "why".
However it was enough for understanding how the parts work together, which tools are to be used for dismounting and remounting, and which parts are to be mandatorily changed at each service, and which parts can simply be cleaned, inspected, and remounted if no fault is found.
At the time I was 18 years old and with very little experience, so I did find the training to be very useful.
People with more experience, and possibly with some technical training done at school or at the university, can easily service their regs without any formal training.
You just need:
1) proper tools
2) service manual
3) original parts to be replaced
All of them can be easily found on the Internet nowadays. In 1977, instead, the only way of accessing to these resources was through the official Scubapro network.