I used this one from Scandvik when I redid my 18SF.
Worked out nicely. I liked it because it was a similar profile (i.e, size etc.) as the original and required no key.
And that is the original location, as far as I know.
At least its the same as my 1975...
My fill hose from there to the tank was a nightmare. I think I ended up pulling it out through the fill hole in the cap while attached to a forklift. It was so slimy and rotten, there was no way I could pull it out by hand.
I put a 4 inch deck plate in the inside of the liner just below the deck fill so I could properly install the new hose to deck fill hose clamps.
You can see it up forward here, on the port side.:
