Signet TK10ML


I recently was able to buy a NOS Signet TK10ML cartridge. It now has about 20 hours of use and sounds glorious. However, I just found out there was a series ll and series lll. I believe mine would be the first series. Does anyone know what the difference would be between the series l, ll, and lll? 

Also, was this Signet basically a hand picked Audio Technica AT-ML180 OCC? 
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