The CL-P1L drop in is the 52mm for the 200-400. I don't know what other filters are available that and the included clear filter are all I have for mine. The clear should always be used (so they say) as the filter is a part of the optic formula for those lenses.
The CL-P1L is different from the clear in that it has a wheel to rotate the filter. If you could fit a standard 52mm filter on the clear holder you would have no way to turn it. I've never tried to remove the existing clear filter from the holder but I don't believe its meant to do so.