"Inch" can be S.A.E. or Whitworth or A/F, though some of those may be the same. If it`s Perkins it`s English related, making metric less likely, but Perkins made small marine engines for Volvo,so the mystery remains. Ask Perkins, or look up a Perkins manual, it should tell you.
At the risk of rounding off the flats on the odd nut or bolt head,there are close equivalents, eg. 13mm = 1/2".
Island Gypsy 36 Europa "Doriana"