Hi therockrt72.....welcome to TFF.
Something doesn't add up here. The Mk.IV, Series 70 Colts all took a single stack mag. In fact, I don't know of any Colt Gov't Model, or any other Colt M1911 type pistol that takes double stack magazines.
I'm tempted to say that what you have is a Mk.IV, Series 70 slide on somebody else's frame....perhaps a Llama IX-C which takes a 10 round magazine....or a Para-Ordnance double stacker.