The above poster is correct. I have to deal with this all the time selling my Full auto glock kits. The LEO has to get a department letterhead in order to convert the gun.
Other than that only C2 and C3's can do it it...Well, a C3 has to get a C2 to make a post sample for him.
I deal with LEO's daily and their biggest hurdle is convincing their chief to sign off on a Dept. Letterhead. This is mainly due to the fact that the uppers don't see a need for that much firepower. I always tell them to tell there superiors that they don't always have their shotgun on hand and with a flick of a selector switch that remedies that problem.
Believe it or not this pitch has enabled plenty of our LEO's to pack the much needed heat in certain situations.