Pictured above is the diamond or lozenge shaped casting mark found on post-production era Ford Model A and B engines.
The raised cast mark is located on the right hand side, at the front of the valve cover, and above the timing gear side cover.
Typically there is also a raised casting imprint of the date tag on the right hand side of the block, below the valve cover. It is not an actual tag or screws.
The actual date tag was screwed to the foundry pattern. The 'tag' on the block is a casting imprint formed by the pattern date tag in the sand mold, and then transferred to the iron casting.
The pattern date tag number is an indication of the casting date of the block. In the example above, the code D305 means April(D) 30(30), 1935(5).
Many diamond block castings were service blocks only, not complete tested engine assemblies, and thus they did not receive an engine serial number.
Some Model A diamond blocks do have factory-stamped Rouge serial numbers, indicating that they were produced and tested as a complete engine at the Rouge. Examples of factory-numbered diamond blocks indicate production dates of mid 1935 and beyond with certainty. I have not yet been able to determine the first use of Rouge numbered Model A diamond blocks.
The as-cast serial number pad was left un-stamped by Ford on service blocks and incomplete and untested engines, as shown above. Subsequently, dealers and rebuilders may have sometimes added their own markings, or transferred the existing numbers from the old engine being replaced.
Beginning in August 1932, complete Model A service block engines were all fitted at the Rouge with Model B camshafts and pushrods (tappets). This feature carried forward into diamond block production.
Another unique feature of many later diamond blocks is the presence of factory installed hardened steel valve seats on the exhaust valves.
There is common belief in the restoration community that these diamond block engines were not cast by Ford, but that may not be correct. The original engine production records from the Rouge (at the Benson Ford Archives) show that these engines were definitely final-assembled and tested at the Rouge through out the 1930's.
The actual location of the casting and machining operations for diamond engine blocks is not certain, but most likely was also at the Rouge, even though the main production at that time was V8 engines, and four cylinder engines were low volume for service only.
I have not yet been able to find any original documentation which explains specifically why this diamond symbol was added to the block, or what it signifies.