In typing this I thought of a reason for it but will continue for any one else's input. I've never seen it before but seems quite sensible now I have a likely treason.
5 amp rated TO3 mounted through heatsink and pcb. One C terminal just bolt and nut , no electrical connection. The other is bolt , solder tag, star washer and nut and the solder tag fully swathed in solder ,bolt hole to wiring hole , merging with thick pcb trace, but only requiring to pass 5amp.I can see that star washer could punch through the thin copper on the pcb so this method avoids that potential problem, and would constitute good practice assembly