+18 psi was for the Mk.50M - the version with cropped impeller. Same for the Mk.45M.It was obsolete by the time 150 PN fuel and 25 PSI boost was available, saying that the Merlin 50 was approved for 18 Psi mid 1943.
Standard Mk.50 and Mk.45 were limited to +16 psi.