I think this was a factory offering in the '80s. Part number 4922 sticks in my mind. As Kenny said it's a marriage of the older 1413/1613, and early 1813 plate with the hook from the 4760 butt.
This is exactly the set up I have on my Anschutz 1813 that my son now uses. I was able to purchase that little part and change it on my rifle. I think back in the late 80's.
I have an old butt hook and cheek riser from a match 54. I was wondering what a reasonable price would be. It won’t let me upload a photo as the file is too large but it looks like the above.