Very interesting, John!
Here are a couple of quick comments about the board and schematic ...
There is a wirewound resistor, thermistor, and an interesting diode, maybe Zener ... all annotated in the pic below. I assume the coil L1 is the WW resistor. I only see two capacitors on the board, but the schematic has four ... could the 'extra' caps be the thermistor and diode/rectifier?
Here are the datasheets for similar lines of thermistors still currently available from Allied Electronics:
http://www.alliedelec.com/Images/Products/Datasheets/BM/GE_INFRASTRUCTURE_SENSING/GE-Infrastructure-Sensing_Actives-and-Passives_8372078.pdf
http://www.alliedelec.com/Images/Products/Datasheets/BM/GE_INFRASTRUCTURE_SENSING/GE-Infrastructure-Sensing_Actives-and-Passives_2391002.pdf
Here is an earlier thread posted by Zendalar concerning his '72 tach, but not enough pics to do a circuit diagram:
http://forums.pelicanparts.com/showthread.php?threadid=179038&highlight=170+tachometer
Here is a pic of the '72 tach with a three-transistor [plastic BC170C] circuit, still using a thermistor and ww resistor, but apparently a 6.8 Volt Zener has been added for circuit stability reasons:
Annotated/revised 912 board:
