Bolex light meter batteries are no longer for sale as they contained mercury. There are replacement batteries of the correct voltage (1.35 VDC) available. One such battery, a WEIN Air Cell, MRB400. It is the same as the original battery voltage of 1.35 volts. As it is a different physical size, an custom adapter is included.
Per the battery's printed instructions, it is recommend that you remove the battery protective cover (green or black plastic tab) approximately 30-120 minutes before first use. You can leave on the black plastic ring (if on the battery) to center the battery within the Bolex battery compartment.
The Bolex camera's battery holder is located on the right side of the camera case at the top right corner. The cover plate unscrews counter-clockwise. The adapter goes in first and either way. There is no up or down on the adapter. Its only purpose is to take up the difference in length between the original and replacement batteries.
What does matter is that the battery itself, the MRB400 <u><b><I>must</I></b></u> be put in with the (+) sign on the battery visible. Or in other words, with the positive side (+) outward. The camera's screw in cover will touch the bottom of the battery and make contact on the plus side of the battery. See the BOLEX manual for the procedure to use and/or calibrate the light meter.
When finished filming for the day, we recommend removal of the battery so the Wein cell is not inadvertently run down.