Howdy
It sounds to me like you got an Essex "conversion". For lack of a better term, a fake. The first guns should be marked with a "P" prefix for prototype. Yes all the early guns sold to the public started at 2000 and they had welded frames and slides. The barrels wer colt with a sleeve brazed on and the trimmed to size. The remainder of the parts were normally hand fitted Gov issue. It is possible that it was a special serial number but unlikely. I have built a number of guns on the Essex chopped frame(basically a commercially produced Detonics sized frame). Some of the serial numbers were C40x C58x and C22x. See the trend? Also you said it was an essex frame? If it is marked ESSEX then it is a kit gun built on the chopped essex frame. Depending on how good the smith was that put it together and how good the parts are that he used, you may have a good shooter or you may have a paper weight. In any case if you paid a Detonics unfired price for it, you NEED to find the person you got it from and get your price "adjusted" Feel free to contact me direct if you need any other info. I have delt with Essex before and they are good people. They could run a production date on that frame for you to verify what it is.
Good luck!
Conley Janssen
USMC 87-93