On a cargo tank's required test/inspection marking, which single letter code indicates that a pressure test was performed?

49 CFR 180.415(b)(3)(iii)

Each cargo tank marking is a compressed safety history on the tank itself, so inspectors, drivers, and shippers can tell what's been checked and when without pulling paperwork. Knowing what each single letter means (V, I, P, L, T, K) is a straightforward, testable piece of the tanker endorsement's DOT retest and marking requirement topic.