The Ruger MK3 mag will fit MK4, and MK4 22/45 just change the base plate. I don't know about the Pro mag.

 

Your issue is that it is a ProMag. A mark III and Mark IV magazine are the same part number. I believe that a Mark III 22/45 and Mark IV 22/45 are the same part number. The metal magazine body, spring and follower of all 4 are the same parts. The only difference is the mag base.

I reiterate, your issue is that it is a ProMag piece of crap. Sorry to break it to you. Just buy an original Ruger magazine and be done with it.

 

I can find Mark iii/iv magazines these days and I cannot find one for the iv 22/45 which is the pistol that I have. I will change the base plates out for TK or VQ base plates.

So, can I purchase the next mark iii/iv mag that I find and just swap out the base plate and it will work in my iv 22/45 pistol?

 

So, can I purchase the next mark iii/iv mag that I find and just swap out the base plate and it will work in my iv 22/45 pistol?

AFAIK, yes. Best deal on base plates, 20 for $10 delivered.

 

I just verified. The answer to the question this thread asks is : Yes.

I Picked up a mark iii/iv mag at Academy and brought it home. Exact same mag body, follower, spring, detent/plunger, feed lips, everything but the base plate. So I swapped the base plate from a mark iv 22/45 mag to the mark 3/4 mag and it works perfect in my Mark iv 22/45 Lite

The wide base plate is the only reason the mag for the mark 3/4 would not fit into a mark 4 22/45. FWIW I suppose you could file the left and right edges down on the wide base plate, but it would have to done evenly to keep it centered. And you would lose the feature of the 22/45 that makes the mags jump out of the mag well when ejected. Or it might not even work at all. I’ll experiment for the sake of it and report back.