I will try to send you pictures of the box. The graphics on the ends of the box say NICKEL FINISH, so I'm pretty sure the penciled N is the frame designation. My 357 was a big gun.....despite only a 3 1/2" barrel.I can speculate. S&W had Model 10's with D series serials but that wasn't until 1968. They often used whatever boxes were on hand to ship guns. Perhaps your Grandad sent a Model 10 to them for some repair work in that box because it is unlikely they shipped a Model 10 to him in that box. Is the handwriting the same?
I have seen some shorthand codes that were written on the box by the distributor or retailer that provided pricing information.
S&W didn't write the frame designation on the box. But they did often write N or Nickel and B or Blue on them.