Sir Alektron . . . . .
How about taking my info below and comparing the foil side of the PCB wiring to ascertain if :
Pin 8 is going to the ground / plane negative of the board
Pin 5 and 6 are connected and going to the voltage supply for the chip.
Pin 2 has a cap going to ground
Pin 3 has a resistor and cap pair going to ground
Pin 4 connects to 7 and they are being the analog output of the unit which either
goes to other parts to feed your analog readout RPM meter . . . . or analog to digital converted as a RPM speed readout.
Pin 1 comes in from the engine compartment to feed either mag or photo-optic pulses into the chip.
I HIGHLY suspicion that this is your chip ad it was used a LOT for this application for years.
Your original unit is a Texas Instruments unit and they had a procedural of assigning new chips with an initial SN prefix until re released.
BUT National Semiconductor was already well esconced with this LM2907chip design initially.
Therefore, so much the more reason for a different designation.
Now 30 years later, TI has absorbed them . . .and just about
everyone else.
If you can't read the board, show me a good closeup of the boards foil side.
And just think . . . . if you wanted a new FORD OEM version of that uit, you could be out $327.
TECHNO REFERENCING . . . .
73's de Edd
.....