The good things about a detachable magazine are that you can carry spares already loaded to speed things up, and- depending on firearm- have them in different sizes. Bad thing is you can drop them or otherwise ding the feed lips which means either a bit of careful work with suitable pliers to repair them or, in bad cases, you toss it and buy a replacement.
Except when you're talking about an old or uncommon firearm. In which case there may be no replacement to find(at least at reasonable cost), or the replacement doesn't work. Which is the case here.
A while back I wrote about a High Standard Sport King .22 pistol. Marvelous thing, which came with the original magazine. The owner discovered two things:
Magazines are expensive, and
ALL the replacements he found and tried, did not work. Oh, they fit in perfectly, but the lips were different from the original. The cartridges were not releasing from the mag properly, which caused the bullet to enter the chamber while the case was still at a downward angle, which caused the slide to whack into it and bend the bullet a touch. Not good at all. So since he couldn't find an original, he took a replacement he'd bought and set to work.
The original mag has a 'tab' formed into each side that both guides the cartridge and causes it to 'snap' up as the cartridge is pushed forward, lining it up with the chamber. The replacement had straight lips, no tab. So he used a dremel and cutoff wheel to cut two tabs and shape them as close to the originals as he could. So far, so good.
Original on the left. Now came the tricky part: shaping the tabs. Which, he found, also included some careful shaving on the outside, as the replacement mag had an overlap along that section which made them way too thick & stiff. He finally, after a LOT of work and a number of ruined cartridges, got it right.
He's now put about a hundred rounds through it, and the only hitch is that sometimes the first round will try to jam when you chamber it; all the rest in that load cycle flawlessly.
Says it was worth every bit of it to have a working spare mag.