Great! with Coandas post, his Ventures Vibramute serial #915 with bolt on neck with screw on coverplate helps refine the change to a bolt thru coverplate to somewhere around #916 to #1036. As always, as Deke pointed out, the serial number sequencing was not always in order, but it tells us that approximately 800 - 900 Vibramutes had the screw-on coverplate.

Of course that's assuming the estimate of approx 75-80 set necks produced is accurate.
Thanks guys, I feel now, that the timeline is out there, those of us on the forum can refine and add to it bit by bit, and get a fairly accurate handle on the changes as they occurred. I know it will never be pinpoint accurate, as serials numbering was not always linear during production, but for the most part we can get a pretty good idea when changes occurred during production.