I recently came across an filter that rarely crops up in Leica circles. As it only fits the Apo-Telyt-R 180mm f5.6 lens that is hardly surprising as there are not many of those on the second hand market. It is claimed that this lens was designed at the request of the US Military and was the first APO long lens than Leitz brought to the market, being produced in two versions between 1975 and 1998. In the later versions an E60 filter came into use, and this is fairly easily obtained. Any Series 7.5 glass is rare indeed as it is claimed that it was never put into production by Leitz on the grounds that the lens was intended for use without a filter! This size was obtainable from B & W of Wiesbaden, probably on a 'made to order' basis.The present specimen is in fine condition and only leaves me looking for the lens to fit it to! It is type 010-a simple UV filter.
The question of 'odd' steps in Series filters is rather strange as they seem to have been designed well into the age of metric screw fittings. First there was the Series 5.5 filter which was a real oddity sold for the accessory lenses on the Leica CL lenses, the( German) Elmar-C 90mm, the Elmarit 40mm, and the 40mm Summicron. The Japanese demonstrated that a metric filter in 40.5mm was perfectly acceptable. There was also a Leica Series 8.5 filter used inside the lens hood of the Super-Angulon 21mm where a very slim filter rim had obvious advantages. It also was used by Beaulieu cine cameras.
This is the B & W effort-: