You may use the plain 7-rd. magazine from a Gov't. Model, but it will, of course, stick out ~0.5". Colt (and others?) also marketed a 7-rd. extension magazine specifically for the Mustang that incorporated a wrap-around finger rest; it sticks out ~1.0".
TomSwift's thread is 4 months old now, but I wanted to submit for the record that I picked up a few of the 7 shot mags with the finger extention on eBay from www.nutmegsports.com
I cannot tell you what an improvement these mags are over the stock mag, even for a person like myself with a small hand.
They are identical to stock. In fact, I think they are, like post contract direct sales without the prancing horse. I bought 3. I had to take one apart because the spring was installed backwards. I had been looking for a year and not found them. These are fabulous! Good luck!