{S-Scale List} What is "Proto-64" anyway?

[MACSIR]

Forgive for asking this question, but I must  have been on vacation in the 
rain forest lately and missed the memo. Just what  is "Proto-64" as it 
applies to S standards? Can someone please explain this  new nomenclature to 
me,or is this just a new nickname for an old item?   Thanks   Lee McCarty