I had mine re-calibrated for the 1.5v Silver Oxide batt's.
I have the GIII and the 625 fit. But according to the picture from the op the battery is PX-1a form factor. and the op has the original QL17 not GIII
A good reading on the mercury cell issue and solutions to it would be this text by Frans de Gruijter:
PX1A form factor looks identical. And I have now found a few references, such as this one by Kim Coxon on the RFF (3rd post down), referring to the earliest Canonet QL samples using this battery before the switch to PX625 mercury cells was made.
[Edit: Here is an MR-50 voltage-reducing adapter designed to convert an inexpensive 357 silver cell wristwatch battery into a 1.35-volt form-correct replacement for the PX1A mercury cell.]
I have two Minolta SrT with a modification performed by a technician to make them work properly with ordinary 1.5V silver cells. Very nice and dandy, but you have to send the camera to a technician (this work was performed during a general CLA for both).
I am a satisfied customer of Frans de Gruijter and consider his solution to be the best overall one, being cheap and practical.
And by the way, welcome to APUG!
There were 3 versions of the Canon QL17. Yours is the earliest version. The canon museum page lists it as a 1.3v MP mercury cell, but ive also seen a similar battery listed as a PX-640, which is about double the size of the usual PX625's, but still 1.3v. So a Wein zinc-air cell will work, but you will need a adapter or something to fill in the gap. A wad of aluminum foil will work, or a spring. A 1.5 volt silver oxide or alkaline will work as well, but you will need to adjust the ISO by about half a stop.
Either way, do not stack two 1.3 or 1.5v batteries together, you will not get the correct voltage.
Thanks again everybody. This is great info.