That’s a Registered Receiver gun built on a HK94 by Bill Wittstein of Billistics in Wallingford, Ct. Was probably built in the early 1980’s. Billistics guns are as close to the read deal MP5 you can get. Back in the day German parts for FA conversion were hard to come by. Where others were making sears, Bill was able to get real deal FA trigger groups.
I’ve got one of these guns. The beauty of it is the ability to take all off the shelf German FA parts. The downside is no flexibility.
Others can chime in, but I think the price is a bit high, I would expect $38-42k for a good RR gun. However, a gun in this condition, from this builder is a rarity.