I worked on a bench with a shoulder vise before and liked it for cutting dovetails, tenon, etc. I have been working with regular bench vises and have just used a spacer to prevent wracking. I decided I wanted something a little quicker in use. As usual I put together a prototype from scraps. My prototypes usually end up being used until worn out. This one will be the same I'm sure. It works very good. Not made for all shoulder vise uses, just light stuff. I'm sure others have made similar. This is just my version. Goes in the vise and tossed under the bench when not needed.
Jim