Thanks!
LPN is a Sussex registration which commenced issue in November 1956*, so that would tie in with your date.
*Curiously, as it looks much earlier, 1950/52 or so, and LPO, which is also Sussex, was issued in 1950.
Usually if a county or a city had several consecutive issues they would use them up in sequence, so LPM 1-999 would be followed by LPN 1-999 then LPO 1-999 then MPM 1-999 and MPN 1-999 and so on. Clearly though that is not the case here, as mentioned above. MPM 1 was issued in July 1957 and MPN followed it directly starting in September 1957, so obviously MPM followed on directlly from LPN 999, those sequences going LPM, LPN, MPM, MPN, NPM, NPN etc. LPO 1 dates from January 1950 so is not part of the same sequence at all, MPO 1 being from January 1951 and NPO 1 from March 1952. Perhaps PO was East Sussex and PM/PN were West Sussex, or vice versa!