Yesterday I sent a mail to Gossen, I asked if they could provide me with information about the Lunasix F. I received a reply with a PDF of the Lunasix F user manual : ) I'm pleasantly surprised that they keep data of 30 years old + devices. If anyone is interested in this PDF, PM me & I will forward it.
A couple of points:
1) the meter you have was one made for the USA market, and badged "Luna-Pro F" (not Lunasix F).
2) in most of the world, that meter would have been known as the "Profisix F".
3) while the instructions for the meter say you can use a zinc carbon 9v battery, it is far better to use the alkaline version, not least because they are much less prone to leakage.
4) I never remove the battery from mine unless I am storing it away. I do try to check batteries in my equipment reasonably regularly.
5) the 9v battery connector in these meters is like the ones that used to be used in transistor radios. It is relatively easy to damage them, but also relatively easy to replace them.
6) most of the manuals for older Gossen meters can be found on their website. They are not easy to locate, are sometimes only locatable through the German part of the site and are a!most universally labelled using the " correct", worldwide name, rather than the special, USA only names reserved for that market only.
Have fun with your meter. That version is excellent. You need to be careful to distinguish it from the Luna-Pro SBC/Profisix when you raed up on it, although it does share many things ( including several accessories) with the Profisix.