AFAIK, the majority of 604G's have annular steel phase plugs. The cost cutting plastic plug came along in '76 near the end of 604G's production. Ironically, about exactly the same time period Urei abandoned them because of QC problems. While they weren't cataloged, i'm pretty sure you could get new 604E's into the 2nd half of the 70's. I don't pretend to know if 604E and 604G were produced concurrently or not. But, i suspect they were for at least a short period. They were still offering 605B's and 604G's at the same time in 1974.when they started phasing in the 604G's with the new Tangerine phase plugs circa 1975.
I probably shouldn't have used the word "revision" as it implies a change was made. I think the cones are all essentially identical, and those suffix digits indicate manufacturing lots, time periods, and possibly even various suppliers..As for the 21184 cones there has been 5 revisions of that cone that I'm aware of but could be more.. 21184-2,3,4,8 AFAIK..
IIRC, it was suggested to me that only the first few were used in production of drivers and cone kits, the remainders were in re-cone kits only.
It is certainly possible that cradledorf's E was an early one, and was re-coned at some point.
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