Aloha JP,
From the TRA judging guide, "Air Cleaners. H4 and H6 systems use different filters due to the differences in air openings. The two types are not
interchangeable although identical in outer appearance. Filters are painted black. "AC" decals are placed in the center of filters. Decals are round or rectangular (believed changed from round to square in 1957), dark blue, and
have white lettering. Paper element replacement filters which might provide better dust filtering are not acceptable
for concours showing."