I’ve looked through my previous replies on this and can’t add very much…. The units that continued the name but were completely new designs were introduced in 2002 and were designed by Alan Bradford for PMI. They are clearly recogniseable by the styling revision.
There were two iterations of ‘enhancer’…. the earlier one made use of a rather tricky IC that proved to be not too reliable. I redesigned it to eliminate the chip making it slightly easier to use, but the difference in performance is extremely small and I did not make any ‘song and dance’ about it at the time. Shortly after that we decided to drop the enhancer as so few people knew how to use it or what it was for!!
In short, any of those VC3 or VC1 units that have an enhancer are my design.