Actually, it came out on the 29-3E/629-2E in
1988. 
The endurance upgrades were completed in 1990 with the elongated stop notches, and internal bolt block to prevent the trigger kickback sensation.
Of course G4F is correct, but to expound slightly, I'd add that certain additions of the Endurance package showed up with the -2E, some other additions were added with no marking (either the "E" or an engineering change number) and then they were completely added with the -4s.
Remember that the endurance package was introduced to keep the cylinder from unlocking under severe recoil from very heavy loads. The frame isn't any stronger, nor is the cylinder which, in fact, has larger notches. Your revolver will do anything, shoot anything, and last as long as later models.
This is all correct, technically, but again, I'd expound a little on the very last clause, in that, yes, older 44Mags will shoot any ammo that the newer ones will and this means they'll last just as long shooting the same loads. BUT,
lasting isn't the same as "holding up." See my notes below.
One part of the endurance package is the externally visible elongated cylinder stop notches. The picture you post of your revolver does not have those.
I would not get all worked up about it if I were you, however, as the endurance package was only found necessary to handle the hot loaded and very HEAVY bullets used by handgun silhouette shooters, who seemed to use bullets weighing north of 300 grains at high pressures and velocities.
If you are sticking with anything like Elmer used or less, and almost all factory loads are less, then you will be fine. In other words, enjoy the standard 240 to 250 loads, and if you need anything heavier, get a S&W .500 Magnum X frame.
I can't agree entirely with this, though, for the most part it is correct. I got a brand-new 629-1 in 1985 and proceeded to shoot it with the Keith Load (but with the Thompson bullet instead of the Keith bullet) for many hundreds (probably 2000-3000 a year) of rounds per year. I didn't do any sil. shooting; this was just the fun/joy of youthful exuberance. Some days more than 100 rounds would go out the barrel, and other days just a couple cylinder-fulls. One day my buddy and I fired over 800 rounds when his mom told us we couldn't go shooting until we'd removed the stump from the back yard. 800 rounds of 44 Mag Keith loads not only removed the stump, but also dug a nice divot in the ground where the tree stump used to be...
The bottom line is that shooting Keith loads was enough to give me significant end-shake and the cylinder unlocking and rotating backwards... This is NOT just a problem for shooting heavy weight bullets.
I believe this is why Brian Pearce wrote in Handloader about shooting the older S&W 44 Magnums the same way you do a Model 19 - shoot special-level loads 90% of the time and shoot the magnums sparingly...
