Agreed, and I'd go out on a limb and venture to say it's post Continental Navy, and probably a 1797 officer's sleeve button privately designed and produced.
We often forget that there was no real standardization of these first officer's buttons, except "they must be flat, yellow, and show a foul anchor". Any superfluous additions such as eagle and shield, motto, etc., could be written off the same as the fact that the well known and rare wood backed Navy button isn't yellow, it's silver!
As far as the eagle and shield are concerned, the first mention of these as necessary are very specific and aren't mandatory until 1797; "Buttons yellow-metal, and to have foul anchor and American eagle on same".
The first mention of both the eagle and shield isn't until a year later, 1798, "a shield on left wing enclosing a foul anchor". Clearly this sleeve button is earlier and not close to that well known mass produced anchor in oval over wing design.
Here's a shot of a British Continental Navy sleeve button to compare style.
http://thomaspublications.com/relics/rw-sleeve-links-navy.jpg
