I would love to install 2-piece custom grips on my 5906 and have been wondering if it is possible/feasible to add a back-strap from a 639 or 659 to the 5906 frame? From available schematics, it looks like a drop in but it may be necessary to drill and tap frame for grip screws. Any thoughts?

Take a look at this, Randy. It should answer your question. I put 2nd gen grips on a 3rd gen pistol a little farther down the thread in post #9. And yes, you will need to drill and tap for grip studs and deal with the fact that S&W changed the grip angle between the 2nd and 3rd gens.

3rd Gen grips on 2nd Gen Pistols (with pictures)

John