I finally had a chance to do a patent search. The idea in this case is not to look for the ears as a patented feature (not expecting to find them, and did not), but rather to look for attachments for try-squares and see that the illustrator drew as a 'typical' try-square in the drawing with the attachment.
Dates are not as refined as I hoped, but I lean toward middle of 1870's as a transition point...
Patent 91,892 of 1869 shows a pointed tri cornered reinforcing plate with 3 screws. This patent is for cutting the plate at a small angle, the rest of the square is claimed to be 'commonly available'
Patent 139,821 of 1873 shows an adjustable try-square with a 3 sided reinforcing plate, but it is pointed, and only has 2 screws, however, it can only have 2 screws as one of them acts as a pivot to calibrate it.
Patent 189737 of 1877 shows an attachment for a try-square. Presumably the square reflects what squares traditionally look line, since it is not part of the patent, it is just there to show what the attachment is attached to.
This patent shows a reinforcing plate with 3 screws, but it also has pointed corners, not mouse ears.
Patent 280,398 of 1883 distinctly shows mouse ears with round cornered plate. (This is another attachment, so presumably the square illustrates a common try-square)
NB: Earliest 'classic' design seems to be 18,327 of 1857 (S. Darling (Brown & Sharp?))
There are a number of squares shown throughout this date range that have plain 4 screw mounting with no plate, so this was not a universal design.
