I did several modifications to several of my Mark pistols last weekend. Replaced the firing pins and extractors in several of them, i.e. my elderly Standards and Mark Is, and added a VQ trigger group and bolt to my Mark III. So off I went to the range with two Standards, a late-model Mark I (with the later-version grip; functionally it is a Mark II) and the Mark III. Happy to report that all of the guns functioned at 100% excepting one of my Standards, which stovepiped a couple of times on some cheap Aguila .22, although it fired it pretty reliably. One thing that was interesting was magazine interchangeability. Mark II magazines most definitely will not work in a Mark III (I accidentally tried this.) My Standards use the same magazines as the Mark I (or Mark II) excepting that the knob on the magazine must be on the right side for the Standard and the left side for the later guns (Mark I A100 grip; Mark II). This switch, by the way, is easy to do on Ruger magazines; it only takes about 30 seconds. I, at least, cannot tell the difference between a Mark II mag and a Mark III mag excepting for the color of the eagle on the baseplate. It used to be that Mark II magazines had a silver eagle while Mark III magazines had a black eagle. Unfortunately now Ruger ships Mark II magazines with black eagle bases, so you have to keep track of which is which when you store them. Anyway, it was a great afternoon at the range with the Marks. Nothing beats spitting out .22LR with Mark pistols, IMSOHO. The long-barreled Mark I is a real tack-driver, no doubt due to the long sight radius: The Mark III with the VQ parts shoots like a dream: This old Standard shoots terrific now that I replaced the extractor. I think that Ruger did not manufacture the extractors as well back in the 1960s as they do now. This gun used to stovepipe now and then. With the new extractor it is an absolute shooting machine, eats anything and shoots great. This old Standard is a "rescue gun" that I picked up used for next to nothing. It has "character marks" (scratches) in lots of places and actually looks like it was holstered a lot. It is somewhat picky about ammo. It shoots CCI 100% but will occasionally stovepipe with cheaper ammo. Not sure why.