I would guess this AR-15 since it was a police weapon had a full auto option.
by def ar-15s are semi/auto. only m-16s are select fire.
I wouldn't be so sure of that. If it's full (or burst) capable it more than likely started as an M16. I've been given to understand that when Colt designed the AR-15 they intentionally designed the lower receiver with noninterchangeable dimensions in the area that holds the fire-control group (trigger, hammer, interrupter, auto sear, and selector). That insures that M-16 parts cannot be fitted to an AR-15, converting it to a select fire "assault rifle". The differences are in the diameters (and locations?) of mounting pins and the width and depth of a milled cavity in the receiver that holds the fire-control parts in position relative to the bolt. Even if you had the M16 parts, the conversion requires the re-machining of the lower receiver which cannot be done on a kitchen table.
PS Any attempt at converting an AR-15 to select fire by person or persons unlicensed to "manufacture" NFA Class III items will result to heavy fines and a Federal felony conviction with a long stay at the crowbar hotel.