I also have a bottle of Grand 15 which I to found in my tee-total fathers drinks cupboard after his death 6 years ago. I have never bothered about it, but only last week I thought I might enquire as to its history , and this morning found your post quite by chance on Whisky.com.
On the top of the box it says 26 and two thirds fl.oz and 70% proof. Looking at your pictures it is the same bottle and box. Have you found any more information?
Judith