Swedish M/1935 backpack | Ryggsäck M/1935
From the late 1800s and onwards, the Swedish army was heavily inspired by the German equipment and arms development.(1) This also meant that the Swedish army used a version of the rigid box-shaped knapsack, that most European armies used. E.g.: the Swedish M/1905. The Norwegian army, until 1905 a part of Sweden,(2) had on the other hand since 1895 used (with great success) a backpack with a rigid frame of oak wood.(3).
In 1935 the Swedish army adopted a pack in similar design, although it did have some major difference. I don't know what facilitated this change in doctrine; but as Lavisse noticed, in his Field Equipment of the European Foot Soldiers, the backpack did have greater volume. More volume made room for more equipment - especially important if the soldiers were to sustain themselves in the harsh "arctic" areas of Fenno-Scandinavia. A sleeping bag, shelter-half, sheepskin coat and other necessities were bulkier and heavier than what most other armies used on the continent. The small M/1905 did in reality only accommodate equipment suitable for a summer campaign.
The 1935 backpack has an exposed steel frame, with leather trimmings and leather straps. The straps can be attached to the ammunition carrier, and transfer some of weight of the ammunition, water bottle and bayonet away from the hips to the shoulders. A small leather strap can be tied around the waist, and prevent the pack from hitting the soldier in the back of his head.
The 1935 backpack is lavishly supplied with external straps.
These are for: Blanket (sleeping bag), entrenching tools (Shovel, pick or axe) and a coat.
The lid has an internal pocket, much like the M/1905 knapsack, and I believe for the same reasons.
A change of clothes (socks and shirt) is often needed when you are out in the field, and instead of emptying the entire pack to reach a pair of clean socks, the lid pocket makes it a lot easier.
There are apparently two different versions of the M/1935 pack. The other version has leather trimmings on the pocket lids (all three of them) and the leather tabs on the lid of the main compartment goes all the way across the lid. The model was most famously used by the Norwegian Troops in Sweden (Polititropper) during WWII.
A photo of the first pattern, can be found here: http://digitaltmuseum.se/011024428377?query=Ryggs%C3%A4ck%201935&pos=1
(1): Compare the German and Swedish equipment in: Lavisse, Emile Charles, Field Equipment of the European Foot Soldiers 1900-1914, The Imperial War Museum London & The Battery Press Inc. Tennessee, 1994 reprint (1902 orig.): pp. 14-20 & 56-58.
(2): Until 1905, Norway had a union with Sweden. But due to great political difficulties and a growing Norwegian nationalism, the union was dissolved (peacefully) October 26th, 1905.
(3) Lavisse, Field Equipment of the European Foot Soldiers... pp. 47-48. & 144.