Hi Bob.

You'd nailed it with 1163 MH, just a bit later than you thought. The 1-12 numbering on the 12 hour register indicates a 1972 model year watch, though the configuration first seems to have gone on sale in mid-1971. The serial number between the lugs might help us to narrow it down a bit more.

It looks to be the one that was sold (at full price) alongside the Viceroy promotion watch - look at the hands and markers on this example:

I'm all for sentiment in this case, there is a connection with your grandfather every time you wear it. Enjoy the watch!

Mark