[UK-CONTEST] Contest call secondary locator question

[GM3YEH]

The local club here (Kilmarnock and Loudoun ARC) has recently obtained the
contest callsign GM7A.  We plan to use it in CQ WPX this year.  Having
trawled through all of the relevant documentation, I can't find a clear
answer to the following question:
    Could we use GS7A for CQ WPX instead of GM7A?

I have the feeling that the answer should be NO - but I can't justify this
based on any documentation I can find.  Any comments would be most welcome.

73 de Barry GM3YEH.