It is a standard (local hardware store) grade 5 bolt (3 lines on the head). With it being 1-1/4" long, there is a short solid section, then the threads. By looking at it, the threaded portion is what is at the actual shear point.
Do not use a longer bolt, as the solid section will also be longer and be at the shear point, making that bolt "stronger" than originally designed.
There is a stronger bolt holding the shear flange to the drive shaft, I suspect it is a Gr8 bolt like used in some of the older 2750 designs that did not have the flange for a shear bolt.
This is the image for my BX2750D, Canadian version: (part description below picture)
View attachment 146523
Part 7:
Shear bolt l/4" NC x 1-1/4" w/ lock & nut gr.5 Part # 70060-01075
Part 32:
Bolt Hex. 5/16" NC x 2", gr.8 Part # 77700-04112
Matching nut, Part 33 Nylon insert locknut 5/16" NC Part # 75599-31912
Part 12:
Auger
Shear bolt 5/16" NC x 2-1/4" w/lock & nut gr.5 Part# 70060-01338
Auger bolts may be grooved