Here's an image of two covers, one with NOV and the other with JAN. I adjusted the contrast on the NOV cover because the cancel was faded.
If the VJ was created as you suggested, wouldn't portions of the slug where the NO and AN were shaved down have to be cut off, leaving two smaller slugs with V and J. Assuming a little more than necessary was cut off of one or the other, could that account for
horizontal shifts in the placement of the VJ as the two slugs were removed (for whatever reason) in then reinserted?
For both VE and VJ day covers it woud have been easy for Stark and others to pre-stage by sending blanks (with or without cachets) to the Victory postmaster, since newspapers would have reported ongoing surrender negotiations with Germany (it wasn't instantaneous) and the Japanese surrender was way back on August 15. A telephone call to jog her memory, if needed, would be an easy thing to do.
Actually, I think this was SOP for most Linto Victory, VT covers. Linto (or Stark) would supply Ms. Stanley with covers with generic (not tied to a specific event) cachets. When an event occurred that Linto wanted to make a cover for, Mrs. Stanley was would be asked to cancel a set of covers with that date. When received, Linto would add a two-line cachet relevant to the event. The top cover in the image above is an illustration of this.
There were quite a few "victory" themed cachets prepared well ahead of VE day which contain nothing about the date. By September or October of 1944 most people were certain that victory in Europe was just a matter of time. Covers with these victory themed cachets were sent to Victory, VT and other places and held until victory was actually declared. In the case of cachets which actually mention the date, these were created by requesting blank covers with a May 8, 1946 postmark as soon as victory was declared, and the cachets added later.