They were a good quality side mount "back in the day". Bases were steel and ring assembly (like yours) was mostly aluminum. The base was attached with screws and pins. Side mounts were often used when it was desireable to have back-up iron sights. In case the scope failed you could remove the scope/ring assembly and leave the top of the receiver clear. I have one of these mounts on a 1903-A3 sporter in .300 Win. Mag. that has been on there since the early 60's and it still works fine--I just shot it at the range a couple weeks ago.
As far as value, I can't imagine it would be very high except to some one who has a rifle with a base attached and is looking for a ring set for a vintage scope.
Very few rifles were factory drilled & tapped for side mounts; they were used back when many rifles were not drilled and tapped for receiver-mounted scopes at all, thus if you wanted a scope on the rifle, extra holes were needed.