Looks like a model 250.
The split-key is not really that.
One side keys the transmitter. The other side opens the squelch of a FM business land-mobile radio.
To make sharing a channel less annoying, all the radios on your "dispatch radio" license will have a fixed low-pitch audio tone modulated at a low level on the carrier. The receiver's audio circuits filter out this low frequency. Hence the name "sub-audible tone" squelch. A narrow filter in the receiver detects the correct tone for your radios, and keeps the squelch closed until one of your units transmits. Each separate license on a channel would have its own unique tone frequency. The other 3 plumbers on your channel won't break the squelch when they transmit and your receptionist won't have to listen to them.
Just one problem. How do you know the channel is clear before you transmit?
To prevent stations from keying on each other, the rules said you have to disable the tone squelch before you transmit, and wait your turn if the channel is busy.
And that's what one side of the split switch button is about. To disable the tone squelch so you can hear that the channel is clear before you transmit.
Mobile radios would use the hand mike's metal "hangup" button connected to one wire of the mike cord. A ground wire on the metal mike-hangup hook would turn on the tone squelch while the mike was "on the hook", and disable the tone squelch as soon as you picked up the mike to use it. And that's why you don't find mobile mikes with a "double key".
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