With the casting date of 1959, it's almost got to be a replacement block designed for a dealer or aftermarket engine change for a worn out or damaged original engine. Quite often, those replacement blocks came with a blank pad behind the distributor so the shop could duplicate the original engine number. That was important in states where a vehicle was titled using the engine number, not the chassis or body number. In that case, the casting number will be for the 54 engine, but the casting date will be 59. Most likely, it will be machined to accept the later model main bearings, and will NOT have shims between the main caps and the block.
Jerry
