Ron, I have a pretty good guess as to why you were shooting low. Your grip angle as well as you rear elbow angle is different with that stock. Has it been a long/longer time since you shot with a thumbhole stock? They do make a difference in your support structure. And with your practice of late being with a non thumbhole stock, you are holding your body differently at the time of the trigger depression, making your shots pull low because your timing is goofed. That rifle may just be a tad slower on firing as well which could make you shoot low. You may not want to shoot your hunting rifles that much, but the fact is, THAT is the tool you will be out there with so it is THAT tool which you should be gaining the deepest relationship with.

Edit to add: Nice looking rifle BTW!!