L37 was Nikon's clearest, least-tinted "UV" filter, sold mostly as "lens protection". The L37C was the newer, multiple layer anti-reflection coated version. The L37C is better, because it is less likely to cause added lens flare/back-reflections than the old single layer coating. I consider the L37C still a viable clear filter for those times when you want one.
The Nikon NC ("Neutral Color") filter is the current equivalent product.
Nikon did make at least one "thicker" UV filter they called the L39, that has a distinct light brownish color. Great for making a smoggy day look smoggier!
