Why does my Shelter 501 have screw threads not bolt channels?


I bought a used Shelter 501 mk 2 here on Agon. The box said 501 .The instructions said 501.The top of the cartridge has threaded screw holes rather than the open channels I have seen it on every picture of a 501- where you need to use an nut and bolt  to fix it to the headshell. So my question is: is this really a 501 or some other model which uses a different fixing system? The pictures of the 901 have threaded holes.... How can I tell.....?

rrm
Consider yourself lucky.  I'd rather have the threaded holes, any time.
But to your point:  I just bought a Shelter 501 Mk2 MONO cartridge in Japan, at a store that had many versions of the 501 (both mono and stereo) on display.  I was at first dubious or at least surprised to see that some of the Mk2 versions (mono or stereo) bore the notation "Mk II" on the cartridge body, and some did not. Likewise, the word "mono" was not embossed on all the mono cartridges. Further, there is now a Mk III version of the stereo 501, as you probably know. Yet "Mk III" was not written on the body of stereo Mk IIIs, necessarily.  So I am guessing that the presence of the captured nuts is yet another production variability.  I wouldn't worry, especially since the threaded holes are preferable, IMO. (I still haven't opened the box, so I don't know whether my Mk II mono will have threaded holes, or not.  I expect not.)