I had the chance to buy one of these not long ago. From my research at the time I figure it's about a $10,000 gun.
I saw one other while researching that did have the letter from Colt. It's serial was a few thousand away from yours and it was listed as shipped to the Russianicon government.

By the way, I can see what looks like active rust. Nice, original, rare...it still needs cleaned and oiled periodically.