Since there is noway to look and the vin. and verify if the origional owner install a rt. tail lamp, go ahead and put one on. Who's to say it didn't have one installed at the dealer. Just make sure the mounting bracket is correct or you are BUSTED!
For me just use one-- we know for sure it had at least one.
But then again I am going to put 2 on a 41 and change the wiring to make the tail stop and turn lamps work with the 2 lamps only.

I didn't do it, no one saw me do it, you can't prove anything! "Bart Simpson"