Hi Tweak!
When I bought my service weapon back in the late 1970s, arranged by my local Chief with our law enforcement supplier, I had to pay the FET as I was the legal owner of the gun even though it was my service weapon.
I recently found the invoice and it was made out to the PD and had my name on it as well. I had to do the 4473, MA FA-10 and pay the LE price + FET + MA Sales Tax to pick up the gun. The FFL was a law enforcement only business and explained to me thoroughly that the Feds required FET on all PERSONAL purchases of LE guns.
When our PD changed guns, the Chief offered me the S&W 64s for a flat $100 each (no tax, no FET) and those were purchased FET-free years earlier with town funds. I did pass on the deal, but I'm sure it would have been no problem. The "one year" rule sounds very likely since it prevents avoiding FET intentionally, but once a gun is 10 years old in gov't service and retired I'm sure that the Feds aren't going to come looking for FET from non-mfrs.
I'm surprised that Scuba was able to purchase a personal gun without FET, but suspect that perhaps the sale was to the PD and later the PD officially (or not) transferred ownership to Scuba. If they did otherwise, the PD or seller may have violated the FET tax law.