You honestly see the terms sort of swapped around on this stuff, a lot of people get confused.
I personally say the base is the bracket that is attached to the receiver. The Mount is the part with the rings and scope that slides on.
There are not many out there where the mount serial, matches the receiver. I think I heard at one time there are only about (8) matching that are known as of right now.
I had a chance at one, one time. But didn't go for it. At the time I wasn't into sniper rifles, but now they are my favorite thing to collect. Go Figure.