Good morning, Mtngael;
There is one characteristic about the Silver-Oxide battery that makes it the best choice generally as a replacement for the old Mercury battery -- the voltage discharge curve as it goes from new to fully depleted or used up. The old Mercury battery had a very flat discharge curve; it stayed at the same voltage as it was used up, right until the very end, where it had a sharp "knee" and the voltage suddenly dropped from its standard 1.35 VDC to going down to the bottom at zero. The Silver-Oxide battery has a very similar flat discharge characteristic, but a slightly higher voltage of about 1.50 to 1.55 VDC. This 0.15 VDC difference can induce an error in many of the older light meter circuits which expect to see a constant 1.35 VDC power source at a very low current draw. The MR-9 Battery Adapter adds a Schottky Barrier diode in series with the Silver-Oxide battery. The normal V(sub f) or Forward Voltage Drop rating of the Schottky Barrier Diode at this current level will be around 0.15 VDC, so it provides almost a perfect compensation for the higher nominal voltage of the Silver-Oxide battery for the light metering circuit.
The only real criticism I have about the MR-9 Battery Adapter is the $40 price. When you have a lot of old cameras, this can add up. One cost effective way to work around this is to just get some small Schottky Barrier Diodes of almost any voltage rating -- we are going to be using it in its forward biased mode in a very low voltage circuit -- and putting the diode in series with the terminal on the battery holder in the camera and the wire that was going to that battery holder terminal. If you can do this yourself, it will save you a lot of money by then just putting in the Silver-Oxide (Ag-O) battery in the camera battery socket with any required additional mechanical positioning ring to hold it where it should be. The anode end of the Schottky Barrier Diode should go toward the Positive or Plus Terminal of the battery, or the cathode end of the Schottky Barrier Diode should go toward the Negative or Minus Terminal of the battery. The opposite end of the diode not connected to the battery terminal should go toward the light metering circuit wiring.