Which battery for a Spotmatic ?

The Spotmatics (except F) used a mercury battery, ref.PX400. It's been ages that mercury batteries are prohibited, and that the PX400 has disappeared. So I regularly meet Spots' containing watch batteries. And as these batteries have no orienting spacer , they are three times out of four upside down (it does not work, and in addition there is a risk of short circuit!).
The battery which replaces the PX400 is called 387S, and is easy to find on the internet (ebay). As this reference equips Swiss luxury watches, there are chances that it'll be possible to find them for a long time. In addition, they are composed of a ref.394 battery (even more common) in a plastic orienting spacer. If you have an empty 387S, just keep the spacer and put a 394 battery into it!

Regarding Spotmatic F, no problem, it's battery was called PX625 when it used mercury , It'still called PX625 in it's alkaline version!

Oh yes, I already hear you say: "Waaah! you kidding us, mercury batteries are 1.35V, alkaline ones are 1.5V, my light metering will be wrong !". And I'll repeat for the millionth time that the zero volt metering ("Wheatstone bridge" assembly) of Spotmatics' meters (and many others) provides a correct measure regardless of the battery voltage!