Thank you for the question, John. I now use The British Museum (Portable Antiquities Scheme) excellent Web site as my reference: http://finds.org.uk:443/romancoins/articles/page/slug/fourth-century-mints I cannot remember what reference I first used, but it interpreted PLON as P(ecunia) LON(dinium) - money produced at the London Mint. As you can see from the British Museum reference they list the alternative Prima (first Officina) for the letter P. Added via Edit (I forget so many things these days, sorry): In RIC, Volume VI, Sutherland theorizes that there was only one Officina at the London Mint. The PLN Mint mark represents a fore-shortening of LON The M series London Mint marks (MLN, etc.) represent Moneta, London.