The federal government was not allowed to sell or transfer firearms to individuals. The Secret Service never carried a Model 66. They carried Model 19, 2.5" blue and had a few Model 19 4". The Uniformed Division (White House Police, Executive Protection Service, etc.) carried Model 15, 4". When I was a local LE in FL, I knew ATF agents who had the 2.5" Model 66. Regarding shipment to BATF, a law enforcement officer USED to be able to purchase a firearm on letterhead from his or her department, stating weapon was for official use, and it was shipped directly to the agency. I know this for a fact because I did it several times. I don't know if this is still permitted. I doubt it.
When we got rid of our revolvers, agents wanted to purchase them. We were told that they would be available to another federal agency or destroyed.
The last thing I will say is "never say never." The privately owned Model 66 in this thread with the US Treasury seal is VERY interesting.