Big Norm,
Crown/RC is the Revision Commission stamp. It signifies a part which is out-of-spec in some manner, but certified by an inspector in the Revision Commission as serviceable anyway. In additional to certifying such a part servicable, it supposedly protected the original inspector from repercussion in case the part failed later. They are not on every part, and not on every gun, and considering their purpose I certainly would not characterize them as "standard".
Erfurts seem to have a lot of them on their guns, I don't recall ever seeing one--or hearing of one, for that matter-on a DWM.
Here are Revision Commission stamps on two different Erfurt frames, noting some problem (unknown) with the parts, and protecting the inspectors whose stamps are adjacent.
As far as the rig in question is concerned, yes, you do see the inspector's stamp--I don't know if it is properly a "proof"--on the stock. It might properly be an Erfurt stock, it is hard to tell if the figure to the right of the serial# is a Crown/Letter or not. There is a lot of verdigris on the mounting hardware, more detailed photos would go a long way to determining whether or not it is an original assembly.
Seems like a lot of money, in any case...
--Dwight