Recently I retrieved a 1926-27 block from my stash that has no serial number and that appears to be unused or very lightly used. I believe it is a replacement block. Apparently replacement blocks came unstamped and the dealer stamped the original engine number on the water waterjacket, sometimes appending the letter "R" to the number.
What I find interesting is the bore size. The standard Model T cylinder bore is 3.750". The cylinders in this block have surface rust but no pitting. After passing a hone through one of the cylinders this is the measurement I came up with.