Your packboard bears a similarity to this on found in the Google Patents database. Check the link and look at the pictures. Note this was invented by a Canadian, and included a tump line attachment. The original design has brackets for the waterbottle and messtin, but perhaps these were removed to make for a more general purpose item. The hooks on your example could have been used for lashing a load to the packboard.
The patent number is 1334582. You can just go to www.google.ca/patents and enter the number.