WAIT A MIN... !!!!
I noticed something else when looking at the photo I have of the 1913 stock book from Remington.
Later stock books used a column just after the grade to record frame size. The first ones were hand written and the even later ones used rubber stamps for the frame size. The 1913 example does not have a column next to grade for frame size. However... they list frame size in the final column under the "Remarks" column along with the shot size and drams of powder, but above that info in the row.
See in this cropped down photo of that page, that the top row actual says "Frame", and then the other rows following just list the number. In the first many examples they are 0 frame guns in 20g. as shown by the bore size a few columns before.
But... Look about 3/4 of the way down when it gets to three rows for 12 bore guns. It lists 2-3/8 as the frame size???
Frame size 2.jpg
What is up with that method of recording the frame size? In this 1913 example 2-3/8, no doubt means 2 frame. But why the 3/8?