Is it possible, given a Cyclone III .poc file, to re-create to some level, the source that generated it? We have an earlier version of the source, but not that associated with the version of bitstream we have.

Theoretically this should be technically possible, given the detailed knowledge of the specific target device, and the detailed spec of the POC file format for it.

That being said, Altera does not provide this level of detail as public knowledge (maybe under an NDA if you were a significant enough customer).

There is no Altera-supplied 'reverse assembler' for their devices as part of the Quartus tools set.

I would suggest you pursue other alternatives as trying to divine the source that generated the POC file you have will be a dead end.