Series (what) adapter

Ok, I've got my nice BL Protar Series VII lens, 18 7/8 front cell.
The inside is thread and measures just above 60mm.
The outside diameter measures just above 66mm.
What adapter do I need for using 67mm filers on this

The threads on a Series VIII adapter will allow modern 67mm filters to screw right in. That's what I used when I had a 4x5. The Series VIII adapter I have has 3 thumb screws that you tighten down around the outside of the lens. It allowed me to use it on every lens I had that was smaller than 67mm filter size. I don't know if it would open up enough to fit on a lens with 66mm outside dimensions. (I'm at work, the adapter is somewhere at home after just moving into a new house.)

I dunno if a slip on adapter is available @66mm if you do find one it may scratch the barrel.
I also use 67mm filter directly in a Series Vlll adapter.