Signetics discontinued manufacturing bipolar proms several years ago, you CANNOT purchase them. Jameco does NOT have them. However, if you have a private stash or a friend with some, build my programmerfor Signetics bipolar proms.

I recommend that you build or buy an adapter socket to use eproms and use an inexpensive commercial eprom programmer with my software to program your radios.

Please NOTE that a prom only needs to read correctly to have a compatible part number such as 2716 or 82S129. Different manufacturers of a part with the same part number usually require DIFFERENT programming voltages, currents, and times. Let me repeat...to program a prom, eprom, eeprom, or eaprom, each manufacturer's devices will probably NOT program reliably unless you use the voltages, currents, and timing that manufacturer specifies! Syntor X radios usually came with a 2816 eeprom manufacturered by Seeq, but double check what you have before purchasing a programmer.

A note on  channel switching - prom address lines A0 to A7 select 32 channels with A3 determining transmit / receive. Additional channels may be added any radio by using more address lines using a small toggle switch with a 2k pull-up resistor to 5v gains additional channels A8(64 ch) A9(128 ch) A10 (256 ch) and A11 (512 ch). You can for example have all A11 be direct talkback (no repeater) etc enabling listening to an input with a single switch. 

Note that Brinkley has adaptors for surface mount in MCX100 radios, as well as standard eproms in Syntor radios. I haven't inspected the former but they look great.