Not sure on this one,Ya might try posting the question (and pics if ya can) over at http://forum.m1911.org/index.php?. Hunter, OD* or some of the others over there can tell ya the straight stuff on it
Just a heads up....they are VERY strict about language over there,If you would have to even think about whether or not to say it in front of a preacher........probably shouldnt say it

I did and here is another question I also have a Colt King Cobra I used to carry ( I was an armored car guard it's a 4 in SS serial number CK 34XX and my wifes off duty pc a Colt Mustang 380 MK IV/ Series's 80 #MU37XX both guns are in about 97% or better what's the value of these)

To get the best idea of what they're going for in todays market I always check http://www.gunsamerica.com and some of the online gun auction places, that would be your best bet(and some local gunshops)

You can call Colt and they can tell you when it was built. As for being rare, I have not heard of any GCNM 80 Series on a 70 Series frame. That's not to say Colt didn't do one as they have done some really off-the-wall combinations over the years. Take the advice given above and visit http://forum.m1911.org/index.php?

Ed66 I'll add my 2 cents, and tell you to go to a gun show also, and see what kind of prices they are getting. The Mustangs,and Government .380s are slipping into collector status. Prices are starting to very big time on them.
Do what Scooter and Bill told you, and you'll have a good idea of how they are selling in your area. Good luck.

I bought a Colt Series 80 Govt Mustang at a gun show. I got it for $525. They are reaching collector status. I wish Colt would start making them again. For a 1911 fan it is a great pocket gun.