Back when the Colt letter was $25 as I recall, early 80's.
My 1860 was identified as 1862 production in a lot of 998 guns purchased by the State of Iowa,
for the 5 Iowa Cavalry.
I think the current cost now is $100-$150.
I have other letters that came with, when I acquired the particular gun, but I think my 1860 is the only one sent for, I actually got. .
The Fire was in 1864 and destroyed only part of the records. I could not get one for my 51 Navy or 49 Baby Dragoon