Joe - I have a No4 MK1 (F) with a 7/48 and a [PF 29,XXX. I believe the F means Fazerkly (sp?) where there is a government arsenal that built/rebuilt the No4s. Mine came into the country via InterArms in Va. a bunch of years ago. I have two No4 MK1s and have played with a MK5 but I don't know what differentiates the MK1 from the MKII. Your metal is better finished than mine and I have the same rear sight. Do you have any clues??? Mikey.