Probably Wizard brand spark plugs...yes they had their own label spark plugs.
Equivalent Mercury Mark 30 specified Champion J6J gaped at 0.025"
Mercury used Champion spark plugs starting with J8J prewar, J7J postwar
and J6J with Mark 30 and almost all other Mark models into early Merc models. More than likely your WA25 would run just fine with J6J IF they were available. You might be lucky enough to find NOS Champion J6J. More likely you will need to use a cross reference to currently available spark plug equivalents which may be a different brand using a different identification system.
Bear in mind that the recommended spark plug is a compromise more or less
satisfactory for most use. After a period of use "reading" spark plugs will
tell if a change to a hotter or colder plug is in order.
Champion 841 = J8C is what I use in early K Models.