Additional information has become available on Remington 51 grip panels.
On the earlier models at least (under PA10000?), the hard-rubber exterior panels are not riveted to the thin steel backing panels. Instead, plate and panel are held together by two short pins, with heads of larger diameter facing the outside, somewhat like the pre-fab head of a rivet. The interior end of each pin has a narrow slot cut around its circumference close to the end, very much like the extractor groove cut around the head of a rimless cartridge case.
The thin steel plate has three holes in it: the holes toward either end are cut in a keyhole shape, with the larger rounded portion big enough to slip over the end of each pin already described. The narrow section of the keyhole fits the groove near the end of each pin. For removal, the plate must be slid until the large holes align with the pin.
The third pin is of smaller diameter, positioned between the keyhole pins. The backing plate, made of spring steel, must be pried or flexed (gently) up (away from the hard-rubber grip panel) until it clears this center pin; the plate may then be driven (again, gently) along the interior surface of the grip panel, away from the narrow keyhole section, until the round (large) openings in the plate align with the larger pins. The backing plate should come free of the hard-rubber grip panel.
The two big pins can now easily be pressed toward their exterior heads to remove them from their seats, countersunk in the hard-rubber grip panel. In some cases, they may simply fall out.
The center pin is very short, and sits in a blind hole in the center of the hard rubber grip panel. It too may simply fall out of its recess if the panel is inverted.
Reproduction grip panels are available from Vintage. Price is above $40 per pair. There is a raised bump in the center of each, on the inside, right where the smaller diameter blind pin is located, on originals.
Regret to report the short small-diameter center pins are not available (see bleow). Later models may use rivets, but definitive information is not yet at hand.
REF: Jim K's post of 03-28-2011, 4:55PM
The grip safety pivot pin at the lower rear corner of the grip is retained by the mainspring retaining plunger. In some guns, it will not be possible to drive the pivot pin either left or right unless the retaining plunger is pressed upwards, toward the top of the pistol, against mainspring pressure. This may require considerable force. A drift punch clamped in a bench vise will help.
REF: duck32man's post of 03-28-2011, 4:16PM
The wonderfully detailed image posted by duck32man showing the Remington 51's cracked right grip panel has a few other items of information to tell us.
The mainspring retaining plunger is easy to see, in the bottom of the frame. It has a cupped hollow where a punch fits in easily.
The cracked right panel appears to be damaged in other ways. It is resting much too high on its frame seat, almost rubbing against the lower edge of the slide. The cutout for the right end of the magazine lock on the panel's leading edge is not very big, but it is definitely above where it's supposed to be. And the grip safety pivot pin (showing its cupped end) is a long way below the recurved lower rear corner of the panel. A correctly positioned panel would have this recurved corner nearly touching the pin.