Thanks for trying. With the few clues I gave it was pretty much impossible to guess.
This is the store bought part:
The custom made part in the first post fits perfectly over the battery switch and prevents you from using the "Both" option on the switch.
I thought it was an "A", "B", "Off" switch until curiosity made me remove the white metal custom piece.
It is an "A", "B", "Both" & Off" switch. Its original purpose in my van was to let you route the solar charge controller output to either the house battery or the engine battery.
However, when I'm parked and plugged into shore power and the old converter is supplying around 13 volts I can turn the switch to "Both" and keep ALL of my batteries at a float charge level. This means I can turn the van ignition key in reverse to power up the in dash stereo when I'm parked and the engine battery won't discharge.
I think the "Both" position is safe enough to use as long as all of the batteries are near full charge. When I first tried the "Both" position the engine hadn't been run in a week and I had been playing with the stereo so the engine batteries were down a bit. I measured 1.25 amps flowing to the engine batteries. That is not much current. And yes, two engine batteries for the diesel motor and one group 31 house battery in this RV.
This will be ok as long as I remember to switch from the "Both" position when it is not needed. That is why the custom made "preventer" was on the switch.
It must has taken a lot of time to custom fabricate that part. There are lots of custom metal parts in the van. Basically if the builder needed a bracket he made it.