By the 1950s and after over fifty years of experience with webbing designs, one would have expected the British Army to be able to introduce a comfortable and well thought out large pack when they introduced the otherwise excellent 58 pattern set of webbing. Sadly the pack they did introduce, whilst having some useful innovations, was poorly thought through and uncomfortable to wear. This led to the 58 pattern pack being universally loathed by squadies and spending most of the time consigned to unit transport. For a fuller analysis of the designs shortcomings, Karkee Web has an interesting article here.
For the description of the pack, we turn to the 1959 Instructions for Assembling Web Equipment Pattern 1958 published by The War Office:
Pack
This is approximately 17 inches wide and 14 inches deep, with 5 inch gussets:
The pack opens at the top and is closed by a flap secured by two straps and buckles. Weather flaps are provided which fold down under the main flap: 
There is a pocket on each side of the pack which is closed by a ‘box’ lid and secured by a strap and buckle:
Attached to the main flap of the pack is a wide strap with a spigot and metal link, and right and left straps with quick-release links and tongue for the retention of the pick handle, or the shovel, in normal ‘marching order’ carrying position:
Above this is a horizontal webbing loop to hold the pick, or the shovel, in an alternative position:
On the top rear edge of the pack there are two adjustable straps each terminating in a flat hook, for connection to the yoke of the equipment:
At the bottom of each side of the pack there is a quickly adjustable strap carrying a spring hook for connection to the rear of each ammunition pouch (or revolver holster) in the ‘fighting order’ of the equipment:
On the rear panel of the pack is a white patch for personal identification markings:
On the top of the main flap two straps, with buckles, are fitted to hold the greatcoat, or the parka:
Two straps, fixed to the top front edge of the pack, cross diagonally over the front of the pack:
and are secured on the underside of the pack by two buckles:
These cross straps are to hold the steel helmet.
These packs are easy and cheap to find so not hard to add to a collection, but there seems to be very few photographs of them ever having been worn in the field.
we used the large pack everywhere i served up until the falklands 1974-80’s bergen’s in ni and self purchased till 88 when i got out used to run 8milers in full kit killed your back and in training with a medicine ball and logs just for fun