I found a vintage "HoneyWell Pentax 3/21 Exposure Meter" and purchased for $25, non working with a nice case and the original instructions.
The problem with these as with other old exposure meters is that the batteries are either unavailable or equivalents are costly and of different sizes to the originals.
This one needed a 1.3 V Mercury cell as well as a 22.5 V Dry cell.

Mainly a hobby project just to get it working again, the meter will be useful with my Graflex, the home brew cameras and the old Nettar folder.

The unit was dismantled and the lenses and body were cleaned with i-p-alcohol.
The optical circuit consists of a 100 mm f/2.5 lens , a semi silvered and greenish mirror at 45 degrees, reticle with a scale, an eyepiece and a Light Dependent Resistor -LDR (Cadmium Sulphide Cell).
The eyepiece (I think called parfocal, not sure in this context) allows all distances to appear in focus, also the scale and meter needle.
Some light goes up at 90 degrees into the eyepiece giving an upright but mirror image, to allow the small 3 degree circle to be positioned for a measurement.
The remaining light goes straight through to the LDR.
Generic LDR have a log10 gamma of about 0.5 and a dark resistance about 1 MegOhm and a bright resistance of 10kOhm of so.
Testing the LDR indicated that it was probably still OK. Testing the meter indicated good, its impedance was 4160 Ohm and full scale deflection was 40 microAmp.

Tracing out the circuit showed that it is 2 separate one loop circuits, the primary one using the 1.3 Volt cell and a set of load resistors, with a microswitch closing if the button is fully depressed to select the 22.5 V battery and another set of load resistors for low light levels. No compensation is provided for ambient temperature or reducing battery voltage.
There were 2 metal battery containers.
The larger one was a good fit for the standard 522 9V battery...... Ahh..So...

To convert the meter for operation on a single 9V battery, a simple one transistor voltage follower circuit was used, using the microswitch to switch an appropriate load resistor in series with the LDR, to replicate the meter currents of the original 2 circuits. The LDR is now sourced from a zener stabilized 5.1 V supply to maintain accuracy when battery is cold or partially discharged. The gain of the transistor allows reduction from 22.5 volts to 5.1 volts in low light mode, as well as acting as an attenuator, at 5.1 volt, for the 1.3 V normal mode.

Circuit simulator "Qucs" was used to optimize the component values.

It probably would not have been feasible to use a single transistor in the early 60's when this thing was designed, due to the lower betas and thermal drift in those days.

The battery case for the 1.3 Volt cell was removed to allow for a small prototype circuit board to be fitted into the space.
The biggest difficulty and time waster on this project was re-wiring the microswitch and getting it assembled correctly in the confined space.

The completed unit was then calibrated by its trim pots using the Pentax k-01 camera in spot metering mode.
After calibration, a comparison was done to compare the modified HoneyWell Pentax 3/21 Exposure Meter with the Pentax K-01 and Pentax MX camera over various light conditions as follows

Object---------------------------------Gray House-----------White Garage Door----------------Concrete Floor indoors---------Cloudy Sky-----------------
Pentax K-01 @ iso 400------------------f/8 & 1/400----------f/8 & 1/640----------------------f/8 & 1/15---------------------f/8 &1/2500----------------
Pentax MX @ iso 400--------------------f/8 & 1/60-----------f/8 & 1/125----------------------Low range----------------------f/8 & 1/1000---------------
Pentax 3/21 Exposure Meter , iso 400---f/8 & 1/250----------f/8 & 1/640----------------------f/8 & 1/10---------------------f/8 & 1/3200---------------

The MX exposes well, it looks like I have calibrated the meter about 1 stop under.
The K-01 was in spot mode, histograms show it is metering for about 127/255, 50% of its saturation.

I put up the circuits in case anybody wants to modify a similar meter
The original Circuit

The new circuit

The case open showing the new circuit board

The finished meter with its original case and document