Unlabeled isolator switch?

[Resolution]

2000 Bavaria 47
There is a single unlabeled quarter turn lever switch under the nav station.  It has two posts on the back.  Two battery cables on one post and one on the other.  I assume this is some type of isolator switch.  Not sure which position it should be in.  We have issues with the house battery bank holding charge, most likely related to age.  Looking for advice on function and correct position for switch.

[tiger79]

2000 Bavaria 47
There is a single unlabeled quarter turn lever switch under the nav station.  It has two posts on the back.  Two battery cables on one post and one on the other.  I assume this is some type of isolator switch.  Not sure which position it should be in.  We have issues with the house battery bank holding charge, most likely related to age.  Looking for advice on function and correct position for switch.

I'd expect that to be the isolator for the domestic battery bank.  It needs to be turned on when you're using the boat.  (Turn it clockwise to turn it on).

[Symphony]

Just to add to that. It operates on the negative for the house bank. You know when it is on as you cannot remove the handle and all the house electrics are live, subject of course to individual switches. Makes sense to turn it off when the boat is not in use as it eliminates the possibility of any drain on the house battery. It is a a very simple system by today's standards and there are now much better ways of managing your engine start and house battery systems.