By the white power plug, there might be a small white
 sticker. That sticker should have the 71xx model
 number. On the botton of the black case, there should
 be a 1" x 1.5" sticker, this will have the FCC ID and
 the Serial number.... the last 4 digits of the FCC
 number is the basic model series ( IE 7172 when its
 knowingly on a 7183 PLUS 2 Radio ), the first 4 digits
 of the serial number is the actual model number.
 Sometimes that big sticker disapears, but the white
 label remains.

 When all else fails, pop the hood.... open up the case
 by removing the knobs and the 2 black screws on the
 case. Slide the case forward. Now remove the 5 screws
 holding the audio logic board, lift gently, it's still
 attached. There should be a small white label
 somewhere on the main chassis board with the model
 number.

 From Dave N9NLU