At least for the 6122 format, I found a somewhat complicated algorithm for determining the EFC from the IR hex code.
For instance, EFC 00169 always produces IR hex 41, but IR hex 41 means different things on different devices. It is the vendor# that makes the difference.

You can tie IR hex#s to functions by using the original remote (or clone) on an IR tester.

My remote did not recognize TV# 14398.

My Western Digital WDTV comes up as the 6122A format on my IR tester.
Its EFCs were given to me in a two-byte format though its lower byte matches my findings in the 6122 format so far.