I used to have a list of 'unicorn' guns... then at one time or another I owned all the important ones. Now I just keep my eye out for something that might catch my fancy.
I still keep my eye out for pre-Model 27's and early highway patrolmans, early j frames, 2nd model Woodsmans,
The hunt ultimately has been much more rewarding than the acquisition.
Here were some of my unicorns over the years...
1st year production 1911 issued to the signal corps (later to become the fledgling U.S. Army Air Corps of the expeditionary forces in WW1).
1902 American Eagle
C96/16 "unissued" reported to have belonged to August A. Wiess, head of Mauser small arms from 1910(?) to 1945.
Mod. 1886 in 50-110
1st year production AR15 (1964)
There have been many more. Some took decades to find, most were gained before the internet became such a misrepresentation of values and before the costs exceeded the worth.
The last unicorn gun I found was a 1st year production Centennial (1952) -- found last year at a gunshow. Guy selling it either didn't know or didn't care about it's low 4 digit serial #.