Those black diamonds are the cubit marks. You remember the story of God and Moses and the Ark? The cubit is God's pwesonal measure ment system and is based on the length of the forearm.
When Horatio Stanley, a very devout Christian, invented the tape measure in 1926, he included these cubit marks just in case someone might need to build another ark.
It so happens that an 8' plywood or gypsum board panel divided by 8 os exactly equal to 19.2" which is pretty close to 19 3/16ths".
This has proved useful in construction since it provides more joists that 2' on center but less than the traditional 16" on center.
~Peter
