Hot Rods What voltage are BLC 856-J fog lamps?

I picked up a pair of BLC fog lamps, but can't find the voltage on the bulbs. I opened one up, but there is nothing stamped on the back of the bulb either. The brass tag is 856-J, and the fronts read "5-3/4 B-L-C Sealed Fog Lamp". Anybody familiar with these?

 

Rubber wiring, I don't have a 6-V battery here, will try that this weekend.

 

good idea, I'll try that.