GS Bergen (75L) (with or without frame)
Arctic Bergen/SAS bergen (120L) (with or without frame)
PLCE Infantry Bergen (OD or DPM)(120L)
The GS Bergen (75L) or Arctic Bergen (120L), commonly called the SAS bergen are butyl nylon, external frame rucksacks and happily replaced the terrible and most certainly not East Wind approved P58 large pack. You will often see sets of P58 webbing sold with the P58 large pack but missing the second water battle carrier, sell the pack and buy a second water bottle carrier.
As alternate acceptable kit, the PLCE bergen with side pouches is easier to come by and has 120L capacity with the added rocket pockets. OG or DPM should both be readily available. There are two sizes for this ruck, short-back and long-back. The long-back is narrow and tall, the short-back is wide and short. The long back is best avoided as it interferes with your belt kit on all but the tallest of the tall. Say no to Web-tex and other cadet grade brands.
The PLCE bergen is designed to work in conjunction with your belt kit for hip support so when you fit your bergen do so while wearing your belt kit.