Looks like a 5-speed frame/tank, and motor - numbers starting with 5 would be a US K5, but all bets are off concerning foreign models.
Appears to be a front end complete from a 4-speed.
the rear fender "looks like" a 4-speed, not sure - it may just be a foreign model fender.
Generally the mechanical siren was mounted in the rear, with the "drive" on the rear wheel.
Never saw anything quite like those shocks.
This was probably cobbled together in-house, not an official police model.