Cap, WW Top Access Diverter Valve, 2", Buttress Thread

The 602-3610 is a specific part number for a white, 2-inch, buttress-thread Waterway diverter valve cap, used by many different spa manufacturers as a generic plumbing component. The actual outside measurement of the cap is approximately 3 1/2 inches. 

Here is a list of spa manufacturers known to use the Waterway 602-3610 diverter cap (or the complete valve assembly, part number 600-3231 or 600-3237, that this cap belongs to) in various models:

This part is widely available from online spa parts retailers due to its generic nature and use across many brands.