"The date code stamp on the oil pan sealing surface is actually the machining date of the block after the casting and before the assembly date"

It would not be for 1962-64. Later year engines, I do not recall as I have not had one apart in four decades.

The image below shows a section of pan rail for a High Performance 289 Cylinder Block Assembly. The 4H25 B date indicates that the raw block was selected, machined, and main bearing caps were added 1964 August 25. The "4H" is was very faintly stamped and can be read easier in a larger file size. In this case the 4 and 4 stamped in overlap are not part of the date code. The engine was assembled in Cleveland the next day 4H26.

Date string:

cylinder block casting date = 4G3, 1964 July 3rd

cylinder block assembly date = 4H25, 1964 August 25th

engine assembly date = 4H26, 1964 August 26th

Also:

Intake manifold casting date = 4G3, 1964 July 3rd

CSX2535blockDATE - Copy.JPG

It is worth noting that cylinder block assemblies used for service work and or sold over the counter 1962-64 had cylinder block assembly dates but not engine assembly dates.