This is a replacement filler cap for the Guardian Equipment G1540 portable eye wash station. The outer diameter is 124 mm and the height is 18 mm.
Yeah, that's crazy expensive for a simple plastic cap, we know. A good incentive to not break yours, isn't it?
Cross-reference: this item is equivalent to the WaterSaver Faucet Company's EW540CP.
2-year limited manufacturer warranty.