The old Leica series vii drop in coloured filters I have were shedding their coating like a powder. The orange one cleaned up perfectly with Silvo then lens cleaner which probably just stripped off all the coating. But the yellow was not a great success. So I looked at a yellow 52mm Hoya and wondered if the glass from that would fit into the series vii holder. The series vii has a screw in retainer that unscrews with a lens tool and some grunt. The Hoya glass pops out once the retaining clip is sprung out. Then the Hoya glass drops right into the series vii holder. It's a bit thicker but the screw in retainer still screws down to flush with the holder edge. Hoya filters on eBay are a lot cheaper than good series vii filters and easier to obtain. Had a look at the series Vi filters and I wouldn't be surprised if the same trick can be done with them.
