This is a rubber washer for a particular type of solar heater (see picture of solar heater... there are many brand names, but if it looks like the pic, then this should work... If not please post a comment). These heaters (or part replacements) are sometimes shipped without the washer/gaskets so now you can print your own.
Note: These are snug fitting as they must hold up against water pressure. You will need to use a fair amount of force, so be CAREFUL not to break your solar heater, hose or fitting. And of course: USE AT YOUR OWN RISK!
Print them with 100% infill (they only use a little filament even at 100%) and also use approx a 1.2mm shell, as well as 1.2mm top and bottom (this actually leaves very little room for the 100 infill, but doing so changes up the printing pattern.
I do suggest a brim, but that is really optional.
NOTE: This print is for personal use only, do NOT attempt to print these and sell them without my express permission!