All British soldiers today call their rucksacks "Bergens" (Often miss-spelt as "Bergans"). The term comes from the original '42 Pattern external 'A' frame rucksack that was closely modelled on skiers' rucksacks made or used at Bergen, Norway.
Elite units, ie: Airborne Forces (SAS, Para, Gliderborne and Glider pilots) and Seaborne Forces (Commando), were issued these rucksacks. Because of the nature of their operations, they were required to be self-supporting and, usually, carry all that they needed to survive and continue their operations independently of the larger logistical structure of the army. Hence the term "Bergen Soldiers", a phrase that seems out of date today. The ethos remains the same: self-reliant, fighting elite men who tackle the most dangerous missions, operate independently in isolation and for protracted periods.
The '42 Bergen came in a few variants and in two or possibly three colours. The earlier ones that the author's seen were not waterproof lined and were khaki in colour. There was a waterproof lined version (circa 1943) then, by 1944, a green waterproof lined version. This final variant remained in service until the early 1980s. There may have been a chocolate brown version (matching the '42 Assault and Battle Jerkins) but the author has neither seen nor heard about any bergens made in this colour fabric.
The '42 Bergen was replaced by the '72 Patt "Rucksack SAS" - also known as the "SAS/PARA" or "Airborne" Bergen. (NSN 8465-99-136-1886) This had a square external frame that was also used for carrying the Clansman PRC 351 and 352 radio sets. From habit, these were called "Bergens" - despite the fact that, strictly speaking, they weren't because of the shape of the frame and pack.
With the introduction of the PLCE webbing, the '72 Patt Rucksack was replaced by the '90 Patt PLCE internal frame rucksack which, a short while later, was replaced by the '95 Patt. The '95 Pattern was identical except for it's camouflage fabric. Both types of these rucks had numerous variants: convoluted backs, quick release shoulder straps, etc, and they come in two sizes, "Long Back" and "Short Back". All of these are standard issue and not exclusive to elite fighting forces.
Cons on eBay:
The main tips to recognising a genuine SAS/PARA "Bergen" from a GS rucksack are easy. The GS is far smaller, has no zipped compartment in the lid, no zip on the outer central pouch and the lid closure straps are buckle adjusted instead of tied. There is no provision for sliding a shovel or skis behind the side pouches. There are no snapper studs on the side pouches (for folding and securing them flat), no accessory loops on the body front and lid and there are two visible 1" straps on the lid, leading to the D Rings for the shoulder straps. Do not be fooled if a rucksack is photographed on the same frame - the GS is designed to be used with the same frame and shoulder straps. With the frame fitted, very little equipment can be carried in the main body - certainly not enough to make fitting the frame worthwhile!
One final point: Originally, all '72 Pattern rucksacks had manufacturer details ink stamped onto the inside lids. On earlier rucks, these were very neat and they numbered several lines. After a time, the "SAS" was omitted from the line "Rucksack, SAS". On some production batches, there is extremely little in the way of markings - and some are "sterile", ie: with no markings at all. Later rucks have markings that appear to have been hand rubber stamped on. The markings, or lack of them, do not necessarily indicate an imitation.
The author has lost count that he's seen bogusly listed GS rucksacks sold as SAS/PARA. Bidders spending £30+ for a £10 rucksack. Ask the seller questions and research what you're bidding on. Study photographs of qualified SAS and Paras (circa Falklands War - loads of pics!). A seller of a genuine Airborne rucksack will try to use the most clear photographs in their listings to avoid any doubts that a bidder might have.
If in doubt, leave it out...