This is Radio Shack's Realistic TRC-431, a basic featured 40 channel AM base station. This radio was the 40 channel replacement for the popular 23 channel TRC-30a  and became available in 1977 when 40 channel radio first became legal.  Like the 23 channel radio, this was a good example of a "starter" base station.  Like on the radio this model replaced, its standard features included Volume, Squelch, Delta Tune, a good sized S/R.F. meter, a P.A. function, an LED channel display, and "On-Air/Modulation" indicators.  The new green "On-Air" light was actually connected to the R.F. output and not just switched when the radio was placed into transmit, and it would glow dimly or not at all if there was a transmitter problem.  Overall performance was decent.  This radio was an improvement over the 23 channel version in this respect.  The receiver was equally sensitive, but the adjacent channel bleed over and strong off-channel R.F. overload problems which plagued the 23 channel radio were mostly eliminated or reduced. This radio also included an improved and, thankfully, a now regulated power supply. The transmitter performed adequately, and sounded good on an amplified mike, although removal of the AMC limiter circuit normally resulted in a bassy, muffled sound, which necessitated some coupling capacitor changes to restore proper audio fidelity, for those who insisted on performing this mod.  

The PLL circuit in this radio utilized the interesting UPD-861, which was one of the first chips which attempted to restrict capability to only 40 channels. Curiously though, this radio did not utilize the "closed" 40 channel-only mode. Instead it was designed to use the "open" 256 frequency binary coding mode of the PLL, which made channel expansions easily accomplished. The bad news was that the radio would only go up to 27.575.  On the other hand, it went far below channel 1, and well into the upper 25 Mhz range. Transmitter power and receiver sensitivity would start to fall off before the point of PLL unlock, so there usually wasn't much push for further expansion.

Like the older 23 channel Navaho,  this radio was popular in our local area in the late 70's and early 80's. Radio Shack continued to be a convenient place to buy CB radios off-the-street, and consequently there were a lot of Realistic radios in the area.  In our local area, owners included; myself, Joe, Wizard, and a few more.

I picked up mine as a trade for work done and used it for a bunch of years, mostly as my "down channels" radio,  before finally selling it in the mid 80's.  On a scale of 1 to 10, I'd rate this radio as a 4. It was a better performer than the older 23 channel radio, but it was still only a basic radio which lacked more "premium" features.