I have one of those 3-stop CFs, made by Schneider and marked 8/90mm. It doesn't appear in the literature in their online archive, but they recently sent me a scan of a 1967 document that lists it. The document is slightly wrong as it says exposure time should be extended 3x while the filter measures 3 stops, but it has the correct physical dimensions. The front threads are 82mm, unlike the later IIIa and IIIb which are 86mm. It also lists filters for the 65mm and 75mm Super Angulons.

Send me an email if you want a copy of this 1.9Meg PDF.