This trial involved only a very small batch of vehicles which were to be monitered closely, I cannot remember now whether bottom rings were left of on building, suspect not. Someone probably recollected a old procedure, from the good old days, when vehicle engines often had skirt rings fitted only after a set amount of miles had been acomplished.
Regarding 330 engines being bomb proof, don't know about that, but in the 22 years I was at REME workshops, our rebuild line was turning them out over that whole period, along with other types. Not often would you see a "leg out of bed", unless they had run out of oil.