To figure out the manufacturing date, look for this group of numbers just below the oil gallery around 6" or so aft of the diamond. Yours will most likely be different numbers than mine is. Month/Day/Year: Month is represented by a letter. "J" is 10th letter in alphabet, so build date is 10/21/1936.
Diamond blocks have some upgrades like hardened exhaust valve seats and a B cam, plus a few spots that were often thin in regular production A motor castings are a little thicker and not so prone to cracking.
Kevin