I've always heard that Mark 2 mags won't fit in a standard or mark 1 also, but I have an old Ruger like the one the OP describes, definitely not a mark 2. I bought it well used about 20 years ago and it came with a mark 1 mag with the silver bottom. I've since bought several brand new mark 2 mags and have used them all interchangeably without modification.
I just checked my guns. My Mk2 is marked "Ruger MK.II .22 cal. Long Rifle". My old gun, which I've always assumed is a mark 1 or standard, is just marked "Ruger 22 cal Long Rifle Automatic Pistol". The bulge in the frame for the mag button is on the left on both guns, but the old one has no slide stop.
Is this how yours is, dirty dave? Are most older guns the opposite?
By the way, the take down is different for MKI mags; just push the follower down with a screwdriver or cleaning rod or something until the button lines up with the hole, then pull it out and push it in the other side.
Also, MKII mags are WAY better than MKI mags, IMHO.