Well the jig/fixture is a platform that supports the router.
To that platform is attached a panel to hold and index the work.
It is essentially a T-section with surfaces ~12 x 16 x 3/4 MDF.
A window in the top panel exposes the end of the stock, (held on end).
The criticalites are the squareness of the construction and parallelism of the support panel. If parallel, edge guides can be used off of all 4 sides to make tenons of any section independent of the work's section or condition.
The router hardware references directly off of the jig. If the jig is well made & the work well prepared, the tenons will be parallel to the surfaces of the sample. The working concept is referencing, secondary, tertiary etc. If the surfaces of the jig are well situated to each other, square and flat, then the work will be well referenced to the work edges of the jig. And in turn, the router, cutter, and edge guides will find the work & rout its tenon faces & shoulders as if directly referenced from its surfaces. (Like a rabbet cutter bearing works directly off the edges of a panel.)
I did have some long (4") arbors, bearings and rabbet bits made so I can also work right from the surfaces of the of the work. (Samples).
So bottomline; it's a platform jig, the work is situated vertically, presented to the router through a window in the jig, and the tenons are routed referencing the work, the jig or both. Any section possible using edge guides or bearings. Plans in an out of print book of mine, "Fast, Easy, Accurate, Router Jigs", a phoney title the editors thought might sell!