These are just the Deck Fill Caps - is that what you are looking for? They are made for the Sea-Dog 1.5" deck fill.
Here is a Technical Drawing of the fitting.
Click on Additional Specs to see the measurements.
Generally speaking, you should put the cap on your fill from the same manufacture as your fill. If you had a metal cap - then your replacement should also be metal.
Ultimately, you need to know the threads of your cap.
For example, Perko and Sea-Dog use different size caps.