PX28 is an old mercury battery designation. You can read all about them here:
but I don't think he can post them overseas. This is also interesting:-
The battery characteristics vary between mercury, alkaline, lithium and silver oxide. In a nutshell they all fade and die in a different way and some cameras will compensate for it and some won't and then you have problems. (And some will leak and attack the leads inside the camera and that will cause a HR and affect the metering.)
I'm sorry I can't help more than that.