old tachometer rectifier(?) components
Combining your pictures with what I can remember about tachs of this era, my assessment is: the object on the left is an inductor, the hidden item is a zener diode, the green item on the right is a capacitor. The 'walkways' are adjustable resistors and the black item in series with one is a rectifier ( round end cathode ). The inductor, series resistor, and zener turn the signal from the points to a fixed amplitude square wave that is passed through the green capacitor to create current pulses. The negative pulses go to the meter and the average will be proportional to frequency. Unfortunately I don't have enough experience with these circuits to offer guesses on the unmarked parts.

I have drawn up a schematic based on Bill's drawing that makes sense electronically (mostly). I have also included a drawing of the waveform at three places in the circuit.

The only puzzling part is the two trimmers; typically there is only one. It could be that VR2 improves linearity. The value of L1 is probably a couple of hundred microhenries up to a milihenry. C2 can be calculated if you determine the sensitivity and resistance of the meter and know the number of times the points close in one revolution.

L1's primary function is to prevent loading down the spark coil ( assuming it is connected there, the circuit could have it's own set of points.) The bit of wire visible in the picture is quite thick implying a fairly low inductance. I would err on the high side and use 470 uH. So long as it is below one mH the reading on the dial should not be affected.

We can guess at C2 based on it's size: it is close to C1 in size so it should be close to .1uF in value. We can calculate the current flowing in C2 by multiplying the charge (Q) in each pulse by the number of pulses in a second. Q equals one half of the product of C2 and the square of the zener voltage: (C2*Vz*Vz)/2. Pulses per second are (RPM/60)N, where N is the number of pulses per revolution. I don't have any knowledge to base a guess about N. Not all of the current goes to the meter, it is split between R2 and R3 as well. After measuring how much current is needed to make the meter read 6000 RPM ( or some convenient amount) , I would multiply it by three and set it equal to Ic2 [(Qf)]. I would calculate C2 and fudge a value for N until it comes to something around .1uF. Since the circuit still works we should be able to determine N by inputting a signal of the right amplitude and a known frequency. If N is one 100 Hz should read 6000, if N is two 200 Hz will read 6000; etc.