Heywood is the Ph.D of the Diana 75, so he will no doubt fix all the mistakes I'm about to make!
But I believe I am correct in saying the true Running Boar version was unique in having a scope rail that ran the full length of the receiver, from the loading port to the rear cap. The RB's also had a unique single-stage trigger set-up to conform to RB rules. This is a very rare and sought-after rifle, IIRC the number built didn't even reach three figures?
The 75U ("Universal") version used pretty much the same stock, and without sights is very similar in appearance to the Running Boar. But the U has a normal-length scope rail, and typical 2-stage 10-meter match trigger. So while a very valuable air rifle in its own right, the U is not quite the rare jewel that an RB is.