Travbrow,
The EPC 100C was first sold in 1976, meant to be sold with the Technics SP-10Mk2 when it came out. The cartridge cost 60,000 yen at the time, which was less than $600. However, that was when the EPA 100 tonearm which was also meant to go with the SP-10Mk2 was also retailed for 60,000 yen. It was not a cheap MM cart - probably costing twice as much as the next best at the time.
The EPC 100CMk2 came out a few years later at 65,000 yen. The Mk3 was in silver in 1981 at 70,000 yen (I have yet to see one of these), and the Mk4 came out in I think 1982 with the SP-10Mk3 when it came out. Each successive version got the cantilever and magnet lighter and lighter, with each successive version seeing lower and lower output. The 100C is 1.5mV, the 100CMk2 is 1.3mV, the 100CMk3 is 1.2mV, and the 100CMk4 also 1.2mV.
FWIW, the EPC-101C came out in 1979 at the same price as the then 100CMk2, and the main difference between the two was the cantilever material - the 100C had a boron cantilever, and the 101C a tapered titanium nitride cantilever. Both sound very nice.
As far as I know, the EPC 205CMk3 also came out in 1979 as a lower-cost version of the 100C. It was a built, I believe, as a cross between the 205CII and the 100C, again with a boron cantilever.
Until this recent flurry of activity, I had never actually seen a picture of a P-mount 100C. The vast majority still floating around are integrated headshell versions (overhang and azimuth can be adjusted, but if the cantilever is off kilter, you're out of luck...).