Router tenon jig

As I've mentioned here, my next project may involve mortises and tenons, a technique I have no experience with. I don't have a table saw, so I figure to cut the tenons with a router.
I knocked together a crude prototype of this over the weekend. It worked decently, but I wanted to make it adjustable for different widths of stock and different lengths of tenon. Holding it in a bench vise made it very stable and convenient.
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