I may be in luck.

I did some searching and found a small spreadsheet someone did for the extinction meter on the King Regula.

It was initially set up for ASA 40 film but black and white film speed ratings were doubled back in around 1960(??). I'll try it with ISO 100 film to see what happens.

There's a labeled ring around the rewind knob (at it's base). Could those be DIN numbers use to tell the photographer what film he/she is using.