Rex Marine has fuel filler caps for a good price. Can y'all advise on the proper size I should order for a 2001 312?

Formula's fuel cap is a complicated piece made up of several parts. The threaded piece (what you need) is a Perko #1270DPG99A,but I don't have a cross reference as to thread size.Just the diameter is not enough,there are many similar caps that are close,but won't work.Maybe Rex can cross reference that,as Perko and Formula have used that cap for years.If you get further info.please post it here