Nathan - I'm gonna through out a SWAG and say that "SPECIAL" casting marker has to do with the engine being part of a Hydramatic drivetrain package (RPO 308 or 314) according to the
1957 Chevrolet Truck Data PDF. I don't have anything to back up this statement, but maybe others have some recollection or knowledge.
The automatic pictured is in fact a Hydramatic and the serial number plate on it should match the stamped number on the block (wasn't pictured so can't confirm) as these were selected/fitted/matched at the factory and installed into whatever truck it was ordered for. The Truck Data file indicates the engine serial# shown decodes as:
F = Flint Plant
I0 = October (10th month)
02 = 2nd day of October
HD = 1957 3100-3200-3600-3800-4000 with RPO 308 (4000) and 314 (3000 Series) trucks
The block casting date I 26 6 decodes as:
I = September
26 = 26th day of September
6 = 1956 (this date coincides with the 1957 Model year)
If someone is looking for a numbers matching (potentially) 235 and Hydramatic, this just might be the ticket.
