Hi Stephen,
As Chris said, the engine mounting bolts are drilled for locating pegs to prevent them turning. Most veteran/vintage machines had the bolts drilled just under the head at right angles to the bolt - resulting in a weakness at that point. Douglases obviously were aware of this problem because on the later machines - like the Dirt track - they moved the drilling to be in line with, and alongside the shank of the bolt - leaving the shank intact.
Regards,
Eddie.