We know the 9N-9600-C1 Air Cleaner was supplied by DONALDSON, used on early 9Ns, alternately with the 9N-9600-C2 UNITED model. At some point during 2N production, the supplier was changed to the C-3 unit, logo was a stamp-raised letter "H" in two places. The top cover screen housing was removable, secured with a wing nut -see picture. Did we ever establish who the "H" or "H-H" supplier was? The 8N-9600 was supplied by ( same, redesigned?) marked with "H-H". I also believe we established "H" was NOT HARCO.