That's real neat, wasn't what I expected, looks to be a manufactured product and not a home made thing,Never seen one before, and been on the scene more years than I care to count,andrew.h.
It's the only one I have seen too, It belongs to a buddy of mine, he got it from his uncle who bought it from a war assets store in the 80's. It was apparently in the large pack when he got it. I always assumed it's a legit military issue item. I'm sure someone knows more about it than I. In the bottom photo, one of the packs has the frame in it, the other is filled with Styrofoam.
I remember being told by a veteran of the Somme that during the Great war Officers marching in full pack with a column of men would actually have an empty bag with a metal frame in it so that they didn't actually have to carry their own kit, but thanks to the frame, it looked like the pack was full. Their stuff would be on a wagon or carried by their Bat-Man.
No frame like that shown in the War Office publication detailing the 1908 Pattern Mills Web Equipment. Ive read of a New Zealand soldier undergoing field punishment at Sling Camp in 1917. He got 4 hours pack drill as punishment and then another 4 when his empty pack ruse was discovered! On the subject of 08 pattern Web,i spent the equivelent of a weeks pay on a full set around 15 years ago,does anybody else on the forum own a set? Wish mine looked as clean as the sets in Sams photo!