If I remember right(it's only been 10 years),you need 2 'D' cell batt's and some commo wire.You run the wire between the 2 phones and when you want to 'ring' the other party,you crank that small crank several times sending an electric current through the wire to the other receiver.The other party picks up the phone on his end and you talk like normal.Secure enough if you can keep someone from splicing your line.I have no idea where to get the wire from though.Shouldn't be too hard to find.
I almost forgot...The fun started when you wanted to gather the wire up.You got an A frame in the back of a 5/4 ton truck and rolled your @$$ off.Also,they only allow a certain amount of splices per mile so if you had too many,you had to cut the line and re-splice the whole damn thing.I just noticed from the pic that some had keypads.I'm jealous! We never had keypads!Oh well...
As I remember also,the current from cranking the dern thing put out a perty good jolt according to some of my commo compadres who tested this on an @$$hole Lt. with braces who was a sound sleeper when drunk.I don't know anything about that though.
31Victor - Unit level communications maintainer, Ft.Sill , OK.