Google "Trim Part Pressure vessel meaning" and the answer is there in the first link.
I am lazy enough to cut and paste the link for you.
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Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches