A correction and some more information. The UHF Mark 12 radios predated the IMTS MK phones, not the other way around. Motorola designed the radios to market to mostly big city departments and capitalize on the newer UHF frequencies. As I said in my other post Boston and Chicago bought them, as did other cities. This move started in the 1960s, not the 1970s. I think it was the early 1970s when Boston moved from a two channel VHF system to the 7 channel system that they still use today, although they've added a few more channels. Their radios had 8 channels, with channel 8 being the Massachusetts Capitol Police. That agency has been gone almost 25 years and Boston took over that channel along with some other 460 channels that agencies relinquished.
As I recall, the radios had on TCXO per frequency with some sort of offset generator (for lack of a better term). They looked nothing like the TCXOs used on the regular Mocom radios.
I got mine when Boston was disposing of all of their older radios in the mid 1980s. They had hundreds of Mark 12s and were dumping them somewhere. I asked about them and was given the two heads and chassis unit. No speakers, no handsets.
If anyone is really interested, I'll take some pictures and post them here when I get back home next week. I wish I had the control head cable and the chassis. Maybe the friend I gave them to to see if the unit could be converted to a ham repeater still has them, but I don't think he does.
If you go
here, you can find some more information and see a picture of the chassis. Scroll about 2/3 of the way down to where it talks about 1968 and the introduction of UHF.
Well I'll comment, and that would be good luck and best wishes. A 12 channel Motrac/Motran/Mocom will be about as rare as hens teeth. I imagine someone with a dire need would be the only one to order some and Motorola would have to SP a design to make it work. This would equate to many $$$.
The reason is for one the cost of putting 24 different channel elements into a mobile designed for one or two freqs. Very expensive.
Reason two is there is not enough wires in the control cable to address 12 channels directly. A BCD switch needed to be used along with a decoder that could output at least 12 discrete lines.
Not impossible but the design group in those days bent over backwards to make a sale. I have never seen a radio of that vintage with 12 channels, but have run across Mitreks modded to something like 12 or 16 channel. Same idea.
So all I'm saying is if you do find what you are looking for may be museum quality. Unless you can piece something together as Steve mentions.
God speed.