MREInfo.com

Basically, the PIR was developed in 1964 by a civilian supply officer, Conrad Baker, who worked for the 5th SF Group in Vietnam. A new ration was needed for the native Montagnards, who were having trouble with their US-suppled C-rations. The C-rations were expensive, heavy, and gave the indigenous troops diarrhea, since they were not use to and could not digest the high-fat, high-protein foods in the ration. NATICK labs determined that it would take up to five years and at least five million dollars to develop a suitable ration.
Mr. Baker then spent $200 of his own money and developed the PIR in his own kitchen in less than eight weeks.
The PIR weighed 22 oz and provided 2600 calories when issued at the rate of 2 packages/day. Packed in a plastic-foil laminate bag, each PIR contained: a heavy-duty plastic bag with about 18 oz of pre-cooked, dehydrated rice; a cellophane envelope of dried fish or meat; a cellophane envelope of dehydrated Asian vegetables; a couple of small plastic packets of sauces or spices; and a vitamin pill. There were 5 menus: beef, fish-squid, shrimp-mushroom, mutton, and sausage.