It has been laying around and I just took time to look at it.
Now I have no idea what it is for
It is the mag on the left

Standard 1911?  Will 45's fit?  Its too short for the DE...

I did not try 45 but 10mm seemed perfect diameter
the reload I tried was a fuzz too long.... very well could be my reload as my S&W 610 shoots them fine

Checked this morning.
Definately a 10mm not 45. The cartridge I used was a bit long, a 10mm in specs fits

It could be for a 40S&W.  The magazine is short in length top to bottom, probably used in a compact frame...

Mag for an officers frame in 40 s&W would be my guess.

post is a year old, It is a 1911 mag for 9mm or 38 super.

Yeah I was thinking .38 super.