Partial VIN on Block

[stllookn]

I know my '68 (original block) only has the engine code stamped into the passenger side block pad and does not have the partial VIN stamped there. When did they start stamping the partial VIN and how much of it did they stamp? I am looking at a great deal on a 327 block and need to know if it is actually came from a later model truck.

Block Casting number 3914678 1965 - 327, 1968 - 302, 327, or 1969 - 350
Front Pad Stamp #1 VO111HM (V-Flint, 01-Jan, 11-11th day, HM...'65 327, '68-327, 302 or '69-350)
Front Pad Stamp #2 18J194659 (1-Chevrolet(?), 8-68(?), J-Janesville Plant(?), 194659-94,659th truck built in the plant...or was Janesville just an engine plant?

I need some help on this second code on the pad...I am guessing at some of those codes since I don't recall how much of the VIN was stamped on the engine pad.