Hi Rickmustang,
This is the text by Bob Gaines that I found here on SAAC:
I believe the transition to the steel version was a evolution of the design . The plastic ones broke and were replaced with metal threaded version. The readily available supply of the plastic version ran out and more and more and were replaced with the less problematic steel threaded version as time went on . This is the most logical explanation IMO . If someone has a better one I would like to hear it.
I hope this helps, I can sell the switch for $600 or trade it against one Cobra Cast Aluminum Oil Pan.
Kind regards,
Mikael