[Arran]

Mike;
I have no way of knowing for sure but this set may be old enough that the mahogany board was finished with shellac rather then lacquer, if shellac it may react differently then lacquer to this stuff. Still I would have a go at cleaning the Howards off, if it is actually an oil based product and not a "refinisher" refinishers are usually acetone based, sometimes with a bit of lacquer thrown in, and will redissolve an existing finish with the idea that you can spread it around to cover the bare spots. If it is oil based you should be able to get it off with mineral spirits/varsol or even kerosene if you have some. Shellac is basically an insect based product, and dissolves with alcohol if you want to test for it, in an inconspicuous area.
Regards
Arran