O.K., here are a couple pics of the almost finished project; need a few more coats of tung oil, to fill the grain. I've had a piece of interesting red African padauk wood for about 20 years. I jigsaw cut two panels and, on a Chinese benchtop mill, thinned them about as far as I thought I dared go, 1/8".
I think they would be even nicer, if I had rounded the rear part to follow the mainspring cover, as S&W has done on the 952...which may be a pending purchase.
I really like the overall effect of the way this came out, because the proportions seem just about right. The 5906 has a fairly short barrel, and the 39-style trigger guard complements my rounded steel spring housing.
That housing, or cover, is the real story. It is hollow and internally welded from six pieces. It could be an investment casting, or aluminum. I bent the back strap from 1/16" mild steel strap, and the two side plates are .115" thick. In the base, I replaced Smith's cheesey plastic piece with one I milled and drilled from the remains of an old hitch from my restored 1941 John Deere "B" tractor. (Try drilling that stuff sometime.)
I use manganese dioxide for parkerizing, to get a nice black color...pretty simple stove-top, non-toxic process which gives a durable finish in about 20 minutes.
Only after I had this finished did I find out that what Smith makes for the back grip piece on the 952 is almost the same. I wish I had the tools and talent to put a diamond pattern on mine, too.
Smith's finger rest mag bottom plate sort of adds to the nice profile, I guess, but a flat plate is more compact. And I have a mag which does that. I found that most Beretta 92 mags are cheap, go right in, and work fine, after a small follower modification, minor bending of the left feed lip, and the Dremel cutting of a new mag release notch.
I wasn't sure how to build this so the new piece would be solid, especially under recoil. But I lucked out. If you fit it carefully, the base beds closely in an existing groove in the 5906 frame, and the top part is easily matched to the cut below the beavertail. Drill the hole in the base for the pin at the right place, and there is no movement of any kind; notice the way the pivot point of the pin prevents rearward movement at the top. That pin works exactly like in a 1911 pistol and looks the same. The mainspring holds it in place and it can be removed in 30 seconds. Nothing else holds my cover on, and it requires no additional modification of the 5906 frame.
Well, except for just one itty bitty nightmare job, which I was not expecting to be a hassle at all.

See next picture.
And here it is with the grip panel removed. The lowest hole in the grip frame is not a mistake, but one used by S&W in the manufacturing process, I think. The other two were drilled and tapped 10-32 by me, but what a nightmare! That steel is less than 1/8" thick, but tough beyond belief. If you don't do everything exactly right, it will destroy your tools and harden, stopping your project dead in its tracks. But with the right technique, it can be done. If you don't know how, mark the spots and take it to somebody who knows. My holes are drilled through, so both sides are placed the same.
This stainless steel is so tough, and thick enough, that there is no need for grip screw bushings, as on a 1911. Tapping 10-32, I have about four full female threads, which is more than sufficient with such rugged steel.
Carefully match and drill your slightly oversize wood panel blanks, using your new frame holes as the jig, and then trim them and round the edges. For mounting, I bought Ace Hardware 10-32 stainless button head, hex type, screws and shortened them. You need a big bearing surface on the wood for the underside of the screw heads, because the trimmed length of the threaded part has to remain constant, so that tightening of the screw, trimmed to final length, will not interfere with the magazine. And because the grip has to be kept thin, so your hand can get around it, you have to be really careful about countersinking the holes in the grip panels, if you want the screw heads to be flush (which is not really necessary, I guess). .050" is the right depth, and I used an end mill, centered to the holes.
The Novak 3-dot fixed combat sight that comes on these ex-police CDNN guns is just fine, but I insist on an adjustable sight on a pistol accurate enough to warrant it, and this minty 5906 is that. This sight cost me about $76 delivered fast, from LPA's U.S. distributor in CT; they have a website.
You can choose three dot, white outline, or plain black for the same price. I've put a number of these sights on various handguns I build up, and I love 'em.
The only thing wrong with this gun was that the paint had come out of the red dot below the safety lever. A couple of minutes, a toothpick, and some red nail polish fixed that.
Oh yes, that nick in the lower front of my new piece is a minor welding defect I didn't feel like piddling with, since the grip covers it; I changed my mind about two unnecessary holes I had drilled, and I filled them with weld. Well, almost.
Am I satisfied with what I bought? Guess so. The second one I ordered this morning should arrive right after Memorial Day.