The Medway Handyman wrote:

And that's assuming you don't have to go round to her house first to
check exactly what size battery it takes. How do you know it's 6v, not
12v, for example?

She quoted from the manual.

I've never come across a 6V alarm. It's probably really ancient and if
you got the box opened you would find a mess of corrosion.

I had one the other week and the alarm was beeping a code that meant
'battery fault'. In fact the corrosion from the leaking battery had
completely dissolved one of the terminals.

Bill