Pictured above is the diamond or lozenge shaped casting mark found on post-production era Ford Model A and B engines made in the mid to late 1930s.
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. It is also found on post-production blocks made in the mid to late 1930s.
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 from Ford.
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. The first use of Rouge-numbered Model A diamond blocks has not yet been determined.
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 push rods (tappets), according to the Engine Production Foreman's records. 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 a 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, though the block casting and machining locations are not clear.
Original documentation which explains specifically why this diamond symbol was added to the block, or what it signifies has not yet come to light. It may indicate the casting supplier or location.
The actual locations of the casting and/or machining operations for diamond engine blocks is not certain, but machining of castings may have been transferred to Highland Park, where Ford performed many machining operations on cast and forged components in the 1930s. The diamond block casting may have been done elsewhere as well. The main Rouge production at that time was V8 engines, and four cylinder engines were low volume and for service only.