I have copies of Schneider's 1977 and 1984 center filter brochures, both of which contain the III and IIIa. The III is listed for the
f/5.6 65mm and 75mm Super Angulons, as
acapela has stated, while the IIIa is for the 90mm
f/8 SA. They are mechanically the same (67mm male, 86mm female) and both are stated as 3x. I assume there's a difference in the filter pattern between the two, but the documents don't give any clues.
The IIIa doesn't appear in the 2007 specs. The IIIb is added, with applications for the 58mm SAXL, 80SSXL, and 110SSXL in addition to the 90mm
f/8 SA. There's also a IIIc for the 47mm SAXL, though a footnote says the IIIb is also usable with that lens. Again, Schneider provides no information as to any differences between the IIIa/IIIb/IIIc.
Here's a link to some info on Rodenstock's filters;
http://www.prograf.ru/rodenstock/acc...n.html#filters
Unlike Schneider, they don't provide part numbers so it's impossible to know if the various 67/86 3x filters are specific to individual focal lengths or if they're "one size fits all".
I agree with Oren - give your IIIb a try. Remember that the lens should be closed down at least two stops from wide open per Schneider's orders.
If you drop me a PM or email with your email address I'll be happy to send you all five Schneider PDFs I've acquired over the years.
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