Does anyone know what this is?
I think it's a ballast resistor.

 

Wow it has been years since I seen one of these guys. They were very common in the mid to late 50's.When vehicles switched to 12V this little guy allowed you to use 6V equipment in a 12V powered car. I have seen them used in 12V conversions of 6V vehicles you could power the gauges and the radio with 6V while using 12V for lighting and the starter of course you had to swap out to 12 V lamps and ignition coil but it was worth the effort for better starting and brighter lights.
They were commonly used in commercial vehicles that had after market 6V equipment installed things like transceivers. The 6V equipment could be put in to
a new 12V powered vehicle with out having to buy all new kit.
I believe you got about 6 amps per leg If I recall correctly.

 

Thanks for the info Matt. Yeah I still don't know if I'm going to keep the original 6v gauges. I saw some retro looking modern 12v gauges from Summit I think that looked pretty cool. If I go 12v I'm still going to keep it on the firewall for nostalgia.

 

What vehicle is this in? if it is in a 57 Ford, the gauges are already set up for 12 V. The Vol-ta-drop is only needed if you are going to run 6 V accessories in a 57. And it might be worthwhile keeping even if not used as it is an interesting period accessory itself.