What a lovely find !!
I completely understand your calling this a grail. The 1163 GMT's are the best of the auto GMT's in my opinion. I dont have one of these red needle examples but adore my own 1st execution 1-24 example ( my favourite execution ).
My observations on the chocolate dials is that all the 1163 GMT's up until the change of GMT hand ( ie from arrow to boxed ) have a tendancy to change colour from black to chocolate. It has been discussed previously & points were made that this fact could be attributed to the black paint used by the Singer factory where the dial was created. Other brand watches using black Singer dials produced in the 1970-72 period have also been noted to change colour dramatically.

Congrats again on a great find - Darren