[Baron Von Zeppelin]

Not a bad theory/opinion
But it skips over the extremely high EUN factor looking at it that way.
In the case of this block.

~IF~ the June heads are original, then the engine didn't leave Michigan until after those dates. Which sort of coincides with EUN it seems like from your other notes.
But the heads are a big maybe either way.

The main thing to remember is the YW block was nothing different from any other 400cid 2 bolt block. Pontiac made a lot of 400cid V8's with this same block casting between the September 68 and June 69 parameters we are involved with. All of them started at the same place in Michigan.
This one didn't get final assembly until way into the final stretch. For whatever reason.
It could have just as well been used to create a YD or YS or WT or WS at any given time. The EUN and application code weren't stamped until it was in the line up to be assembled.

The EUN is pegging its assembly time frame to wherever that 8xx,xxx EUN falls in line.
For an off the shoulder loose comparison, 1968 400cid EUN's hit the 8xx,xxx range in June/July production. fwiw