They were the Cadillac of their day. Our Dept had Instalerts, a not very close second. Dual conversion receiver, with LC tone circuits for decoding a non-standard tone sequence. Most tone encoders used 1 sec A tone 3 sec B tone. Plectron used 3 sec A 100ms B. Mainly to keep Motorola at bay, which Dutchess County still uses to this day, making for a limited number of Motorola products that decode their format. The unit is basically bulletproof, the biggest problems I can remember are the internal battery would leak,(an early pocket pager??) the volume controls would get flaky and their even was a recall on the power plug for some reason I can't remember. I still have a few, one on my Dept's tone and a 1/2 dozen or so another Dept gave me for parts.
Plectron made about a dozen different models including some tube models, some cutting edge receivers for the day, that I used as siren controls, they also tried making a pager, but it wasn't very good and had trouble competing with the Motorola Minitor I and the change to portablity. Unfortunatly they went under around 1983. BTW it appears that unit is/was from Chelsea FD.