For a standard (non-excepted) DOT cargo tank, which two intervals under 49 CFR 180.407 are NOT the same length?

49 CFR 180.407(c)

These intervals are easy to confuse because several share the same number of years, but they test different things: surface condition, structural integrity, and leak-tightness. Keeping the external-versus-internal distinction straight is essential to correctly reading a tank's inspection history.