Don't know much about it firsthand, played what looked like a BR-6 a few weeks ago and wasn't too impressed, my Electromuse sounded lots better.
Did a bit of looking, found one
Here going for $850, and some good clear pictures. Another
Here, no price listed (already sold) but more good pics. Measurements listed too, that may help.
Also spotted another listed for $825 at Gryphon, no pictures.
So it's looking like it's value will be in the $800 range, in excellent condition, authenticating would be more difficult, the only sure way I know of would be to have an expert check it out. I'd say check the measurements and look for obvious signs of age, it's 60 years old and should show it, the only definite way would be to open it up and check the numbers on the pots, they usually are stamped with date codes. My Electromuse still has the original pots in it, the Gibson might too. Being a 1947, same age, it should have a rectangular capacitor inside that looks like a domino, with several colored dots to identify the capacitance.
Unfortunately my original cap was trashed when a bass player caught his foot in the cord and yanked it off the guitar stand while packing up second or third time I brought it onstage. I also had to replace the original cloth covered cord, it got yanked too and refused to work any more.
I'm not sure what else to look for, mainly obvious signs of age like yellowing clear coat, tarnish on tuner gears, yellowed tuner knobs, that kind of thing.