Model 29 was superseded by the Model 29-1 in 1960, with modifications made to the ejector-rod screw. The Model 29-2 replaced it the following year, with one screw that had secured the cylinder-stop spring being deleted. The barrel length was shortened from in 1979. These two versions are known as "pinned and recessed". "Pinned" means that the barrels are screwed in, and secured by a pin driven through the frame and a notch in the barrel. "Recessed" denotes the rear of the bored cylinder holes being countersunk, so that, when loaded, the cartridge rims are fully enclosed by the cylinder. In 1982, the cost-cutting Model 29-3 dropped recessed cylinders and pinned barrels for crush-fit barrels. The -4 and -5, produced from 1988 and 1990 respectively had changes to improve durability for heavy use. In 1994 the 29-6 began production, now fitted as standard with rubber Monogrips from Hogue to replace the previous wooden items, standard tapped holes also being provided for attaching scope mounts. The 29-7 started production in 1998 with changes to the locking mechanism, the firing pin's attachment, and a hammer and trigger produced with a metal injection molding process. Model 629 Introduced in 1978, the Smith & Wesson Model 629 is a stainless steel version of the Model 29. The 629 model designation derives from Smith & Wesson's practice of denoting a stainless steel version of one of their already existing designs by placing a 6 in front of the model number of the original weapon. The 629 Classic variant features a full-length barrel underlug, other variants include the 629 Stealth Hunter. Quiet Special Purpose Revolver Some S&W model 29s were rebuilt by the AAI Corporation to make the Quiet Special Purpose Revolvers (QSPR). These had a new, short, smoothbore barrel length of , with an overall length of , and a bore, in addition to having the cylinder chambers reamed to accept the special QSPR ammunition which externally resembled metal-cased .410-bore shotgun shells, but internally worked as a piston to trap the gases. This pistol was developed from 1967 to 1972 to be used by tunnel rats in the Vietnam War. The QSPR was tested on the battlefield in 1969, and an improvement and testing program ran from 1969 to 1972. It officially never entered service. The US withdrawal from Vietnam reduced interest in the QSPR weapon, and the program ended in 1972 although unsubstantiated rumors