With the early Sig P238s, use of a Mustang magazine was hit or miss. They'd fit but sometimes you would get a failure to feed or something similar.
In the first year of production of the P238, it had several problems which many folks attempted to fix using Mustang springs and magazines. As the P238 evolved in the first year or two, Sig changed the follower several times and finally settled on a flat main spring. (Note, Kimber now offers a flat main spring for the Micro) One difference between the Mustang and P238, the P238 barrel cams down a bit farther than the Mustang changing the angle relative to the frame that cartridge feeds into the barrel.
As Sig magazines became more available, I just bought them for my P238 and kept the Mustang magazines for my mid-1980 vintage Mustang.
After Sig fixed the operation of the early P238s and mine ran flawlessly, I stopped paying attention to what the forums were saying about P238s and Mustangs.
I recently bought a Kimber Micro and have not tried mixing magazines with it. The Micro does not depart as much from the design of the Mustang as much as the P238, but I have not done a detail comparison either. The Micro works with Micro magazines.
Magazines for all three should be readily available these days and I do not think the prices vary very much. But I could be wrong, it has been a while since I bought magazines for any of these guns.
I'd just get the ones designed for your Mustang and be safe.
Colt 380 Government magazines and even a Mustang 10 round magazine work just fine in my Mustang. The only difference is the length of the magazines. I have a 10 round magazine that I used regularly in mine when I shot it regularly.
