The clear filters Nikon make today are NC filters only?
I assume the coating are much better today than the old L37c filters?
They could be quite hard to clean so they looked like when they came out of the box.
Try cleaning L37c filters with regent grade methyl alcohol as recommended for Nikon's view camera lenses in their instruction manuals, Nikkor-W, SW, etc.
Multi-coated Nikon filters are no harder to clean than single coated. Look closely at the single coated filters. The swirls that are easily seen on multi-coated filters are there on single coated filters. Those swirls just show up better against the less reflective multi-coatings. Kodak Lens Cleaner was notorious for leaving smears on lenses and filters.
I've used regent grade methyl alcohol on vintage 1969 and later Nikkor lenses through the AIS lenses as well as Nikon's view camera lenses. Your Mileage May Vary!
I've smeared the blacking around a '70s vintage Schneider 135/5.6 Componon-S all over the glass but cleaned it up and I re-blacked the edge, no harm, no foul at least functionally.
Dave