Based on customer feedback & some personal use, the ENDO frame helps with structural & lateral stability with heavier loads but isn't a "game changer" in that regard. The previous version of the Gunslinger II used a plastic framesheet for structure, and some owners complained that the pack was unstable when hauling large, heavy loads. The ENDO was designed to lay inside the pack & be held in place by edge contact (with the pack's Cordura) and the gear inside the pack. While it does help in giving some edge to edge structure, it can't provide complete stability because of it's location (inside the main compartment with the gear). It doesn't help with centering the loads over, and closer to, the hips, which is the best solution to making the pack more comfortable with heavy loads.
The newest version of the Gunslinger II now uses the Intex II frame, which is located at the edges of the pack closest to the back padded area. It is designed to help with lateral stability AND keep as much of the load (as possible) centered over the hips. It isn't perfect, but it greatly helps in limiting the lateral sway that some experience when trekking heavy loads over uneven terrain. At issue is the weapon scabbard's size & location, which makes it both a design feature AND a design flaw. It is a cavity that, inherently, causes the load to lose it's center over the hips, yet it allows the user to comfortably haul their chosen weapon "hands free" over rough terrain AND have it still be accessible for quick deployment. There is a trade off here, do you want a pack that can haul your weapon protected and allow your hands to be free, or do you want a mountaineering-designed pack that uses a design focused on centering your load over your hips (at the expense of having to carry your gun either in your hands, or tethered to the side where it is exposed to the elements, obstacles, etc)? No right or wrong answer here, it all comes down to user preference & applications to be filled.
It sounds like you'd be getting a smokin' price for the previous version of the Gunslinger II (without Intex II frame), so that may be your best option if you're looking for a good deal and a pack that does a lot of things well. However, you're not going to get the same stability with heavier loads as you will with the updated model that uses the Intex II frame. A LOT of guys here could care less their Gunslinger II comes with an Intex II frame or not, as they don't forsee the need to pack heavy loads over wide areas. With weight up to 50 pounds (give or take, depending on what is being hauled), the old pack with ENDO will be fine. If you're looking for a pack to pull trekking duty with large loads several times a year, I think you'd be wise to go with the new model with Intex II frame. I think you'll feel a difference between the two...
Hope this helps & best of luck with your decision.
Ceylon
deltaoutdoorsman.com
EDIT TO ADD--the post above was submitted while I was typing out this response. I have the new Gunslinger II with Intex II frame at my photographer's studio now. He will be getting close up, hi-res pics of the new pack that will hopefully answer some questions regarding the new design. The thread link referenced in the post above should be updated with a complete review & pics by the end of next week (he should be done taking the pics by Tuesday the 27th, and I'll need a day or so to put my thoughts together)...