Maybe like this one: Jansport Cascade Internal Frame Backpack, circa 2000
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Oooh! You're clever! I believe that would be another clue if it didn't come out until 2000. Some stores would have had it in stock into the next season or the season after. It's even possible Jansport knows where those packs got shipped, since I doubt it was stocked in some of the more conspicuous stores, like REI, because it was getting to be a little "old school" by then. There's plenty of room on a pack that size for what the UID had: you can see from the photo that a sleeping bag is strapped to the outside. The photo you provided even gives some ideas about how he would have packed it.
That is a 70 liter pack: biggish, especially if you carry stuff on the outside as used to be common.
That "sleeping bag sack" may have been used for clothing, and not a sleeping bag. Personally, I never squeeze my clothing into a tight sack, since the sack ends up like a big lump and is hard to find a spot for in a pack. I pack my extra clothing (such as I have on a multi-day trip, lol) loosely in a larger bag and then plop it in my pack in the crevices.
It is not that rare these days to not take a whole lot of clothes on a backpack trip why stink up 2 sets of clothes instead of one?but it is very noticeable in this case that there don't seem to have been any insulation (e.g. puffy jacket or fleece), rain gear, or spare socks. You just don't go out there without those items in the mountains, even almost on a day hike! If the guy had a good tent and pack and long underwear, not to mention a top notch filter, stove, bear spray, and maps, he would have had the right apparel. I mean, really, he had a "tent repair kit." (I don't even have one of those. Interesting that he had a tent repair kit and no first aid kit.) Even if he were a hobo, he would have been entitled to some kind of warm jacket at a nearby homeless outreach. I don't get this at all.
Either someone stole a down jacket, rain gear, a headlamp, hat (and maybe a pair of socks), or the police missed something on the scene. ( If the money was on the body, it might have creeped out a thief, and was not touchable.) Plus, there will have been food, toothpaste, et al. (maybe a first aid kit), either hung from a high branch some distance away (most likely) or they were in a bear bin tossed on the ground (less likely), also some distance away. These will have been very easy to miss, if the police were not familiar with backpacker protocol.