I have actually had my sticky paws on one of the "failed" Lithgowconversions.
The reason if failed was that it was subjected to a diet of OILED proof rounds. That is a good way to seriously stress ANY action. The "blown" action had also been modified to accept FN/L1A1 magazines.
The original project was allegedly intended to produce a "second-line" rifle, with the L1A1 being general issue.
I suspect that whoever organised the oiled proof rounds had no intention of allowing a "second line" rifle into the system.
As for the materials used, I think I have posted the steel recipes from the "early" days, and 1938 here previously. I can dig them out again and let the metalurgists comment on the differences.
Is was an article of faith among target shooters that pre-1919 Lithgows were the "best" metalurgically speaking. This may have been related to the proportion of "vintage" Lithgows used as platforms for the "T" series.