From Confidential Service Memorandum MG/259, dated 29 June 1959:

VALVE GUIDES - REPLACEMENT ENGINES

A small percentage of .010" ( .25 mm.) oversize valve guides is fitted during assembly of replacement engines. These can be identified by VG+.010" stamped on the cylinder head adjacent to one of the stud-holes where it is not obscure by either a nut or a valve rocker cover gasket.

Replacement .010" oversize valve guides can be supplied (see Parts Lists). No other oversizes are available.

They can be identified my a shallow groove in the outside diameter.