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.....?