Just carrying on the theme of the ancestry of current rations, this I would say is the direct descendent of the current MRE concept, as it seems to be the closest in size of sub - ration (the meal) and approximate colorific intake. Each FPIA had a minimum 900 cals. Even the weight of the meal at 1.5lbs is similar to the MRE Obviously the packaging, steel as opposed to composite retort material is completely different.
It is the 1948-49 FOOD PACKET, INDIVIDUAL, ASSAULT.
I think this is closer to the MRE than the C or MCI series.
its funny but it seems that Natick were not exploring the potential of the "Meal unit" and were still hung up about complete rations. From the contemporary description above, it seems they were very cautious about its duration of use. However looking at its components, it has all the elements of the modern MRE - Entree, crackets, dessert type item and an acccessory pack. With a little tinkering of the items the US army could have had a wide ranging MRE type unit in the 50s

It seems the 24 hr ration type was still in favour at Natick at the time though.
I know the MCI was a meal based unit, but it was 10 years later, and 2 times heavier.
I reackon this is closer to the MRE.