Length:Length is measured from the outside edge of end cap to end cap, not including any handles, threads or slip fittings. 4-5/8"
Bottom:This is the actual inside hole diameter, excluding any taper.1-15/16" Open
Premium replacement for:Unicel:Unicel part numbers either start with a "C" or contain a "CH" like "C-8475RA" or "6CH-49".C-4302
Pleatco:Pleatco part numbers start with a "P" like "PWW100ST".PSS17.5
Filbur:Filbur part numbers start with an "FC-" like "FC-0360".FC-0183
I ordered 11 filters for our hot tub. They are the same model number as the ones I ordered last time but they DO NOT fit. They all measure 1/16th of an inch smaller around than the ones we still have from our last order. It's pretty obvious that they not of the same caliber as the others so I assumed you got them from a different company. If you would like pictures I will be glad to email them to you showing us measuring them and you can see the difference. Please get back to me on this matter:-(