SKPC
Cruisin' on my Bluebird
While some may be aghast, I would simply round the sharp end of a framing nail, place the hub protected by a rag in a vise (lightly) heat the retaining ring with the spanner holes, then with a small hammer and nail punch at the proper angle, lightly tapping either side to unscrew? Must be right hand threads like most Sturmey/Sachs 3-spds. A good mechanic that tightened it could have assembled it correctly.