The oldest CI version in our code repository is 2.0.3, from Jan 2011.
The last version from the CI1.x version, according to the changelog, was 1.7.2, from Sept 2009.
We don't have a record of CI 1.7.3 in our repo, though I do see references to it when googling.
It would appear that CI1.7.3 was not officially released. The "CI1.7.3 references" I found searching would appear to be part of CI2.0.0, and they seem to suggest that there was an unofficial packaging incorporating some of them.
There are some stackoverflow and other (not recognized) threads about "1.7.3", but I don't know how much faith to put into them.
Given the age of your codebase, it might be better to look at rewriting it in CI3 directly, rather than proceeding through each of the versions since then.
