This question relates to ERS H.1.1 This is the clause that, in short form, says…
An owner wants to put some kind of identifying mark on his IOM sails to help identify his boat during close racing. Is there is anything in the rules preventing him doing this?
Yes.
The IOM class rules are closed class rules – to quote:
The class rules for the International One Metre Class are closed class rules in which anything not specifically permitted by the class rules is prohibited. Individual rules may require, limit, or permit as necessary.
Of the Section G rules (applicable to sails) the only clauses relating to the construction of the sails are G.3 and G.4. None of those rules permits marks or decorations on the sails other than sail shape indicator stripes which are limited in number and width.