Endurance Package is usually veiled under a foggy layer: someone sometimes talk about it, but nobody knows exactly what it is.
Well, I'm too curious about our revolvers, and I make always several attempts to discover their "secrets".
The package was introduced to solve some issues of M29 when stressed by huge reloads and rapid fire.
Modification was applied to N frames, and it is of course easier to keep it in manufacturing ALL N frames, even in others models , as M27 . To me is difficult to explain in correct English: for this reason please accept a cut&paste from an American website.
"In 1989 S&W began to add the Endurance Package to both the 29 and 629. Using high-speed photos, Smith & Wesson engineers studied the .44 Magnum under recoil and concluded that problems arose as various parts moved under recoil. The frame recoiled backwards and the cylinder pin did not. The result was the cylinder was free to rotate. Heavy recoil was causing the trigger to kick back and the hammer was bouncing and hitting the primer a second time resulting in two indents on the primer. The engineers went to work. Longer notches were machined in the cylinder so the cylinder stop could not bounce out under heavy recoil. A bolt block was added that keeps the bolt from transmitting movement to the trigger. All mounting studs for rebound slide, trigger, hammer, etc., were radiused where they attach to the frame as round corners are less likely to "tear" then sharp corners; all receiving holes were also radiused for the same reason. To help increase strength, the bearing surface on the cylinder yoke was increased and the yoke also received a new heat treatment. All of these improvements were phased into the S&W .44 Magnums in the late 1980s and those with the complete package have a noticeable longer bolt slot cut into the cylinder."
Other source : Silhouette shooting was popular some decades ago, and heavy bullets went in use and issues began.
"When 300+ grain magnums were fired in .44s that had end shake frames were recoiling back away from cylinders so far that the front of the stop notch depressed the locking bolt allowing the cylinder to rotate the wrong chamber into line with the barrel. Long stop notches were S&W’s cure. The bolt block prevents the cylinder latch from moving back under even heavier recoil. It ended the problem of cylinders flying open."
I hope I've been of some utility to you.
