I stole below from Vietnamgear.com so if you want to see a little more go check them out.
Development of the Tropical Rucksack (a.k.a. Jungle Rucksack) began in 1965 when MACV requested a nylon version of the ARVN pack from Natick Labs for evaluation purposes. However, after initial testing the rucksack had to be significantly redesigned in order to strengthen the pack and round off the corners of the frame.1 The redesigned rucksack was assessed against three Cresson H. Kearny modified Gurkha-Australian packs, before being finally approved for procurement in early 1968.
The Jungle Rucksack was slightly heavier and more expensive to produce than the Lightweight Rucksack, but offered quick-release buckles on both shoulder straps, greater carrying capacity and fewer projections to get snagged on the undergrowth. In addition to replacing the Lightweight Rucksack, in January 1968 ACTIV recommended that combat / combat support units in Vietnam be issued the Tropical Rucksack rather than the M1967 butt pack.
The Tropical Ruck is a pretty cool pack and I am stoked to add one to the collection finally. The fame and short back of it make great for hot weather and combining it with belt kit. Interestingly it seems to have been popular to mount the Tropical Ruck onto the frame of the Lightweight Rucksack.
The Tropical Rucksack bears a strong resemblance to the medium ALICE, no surprise considering it is the direct predecessor to it. There are a number of differences naturally, the biggest one being the frame style. The ALICE main bag is also a few inches larger in all dimensions than the Tropical but the external pockets are roughly the same. Also there is only one horizontal strap location on each side versus two. There are more but those are the readily visible ones.