Both are Infantry Bergans (not to be confused with the other arms "combat handbag") with a capacity of 100 litres. One is meant for people over about (ISTR) 5 ft 8, and is taller but thinner, hence "long back", the other is for shorter people and is shorter and fatter (and easier to stuff an issue gonk sack in), hence "short back". The way to tell the difference is to count the padding panels on the back. ISTR the long one has 4, the short one 3 (I can check this later be counting the number on my long back). As I should reemphasise, both have the same capacity (100 litres).
People are confusing the short back with the other arms Bergan, which has a much reduced capacity (something like 50 litres ISTR, but I've never really carried one).
I noticed a Picture the other day of a "Full Sergeant" In the House Hold Regiment/ Scots Guards Devision. The SGT had a "Pace" stick with him, I thought the pace stick was for CSM/RSM? Can anyone explain, Or is this a Tradition of the Guards,?