What Is This Metal Detecting Find? - En Union Y Libertad Button

Recently ScrapIron was out metal detecting in a site that was once an old river crossing throughout the 1800's.  The site was crossed by horses and wagons for a long time and is near where an old mission used to be located.

He was detecting with his XP Deus II metal detector and got a decent, but deep, signal.  After unearthing the small round item he realized it was a rather small button.  Unable to make out what it was it in the field, he put it in his finds pouch and moved on.  In the same area they found several old musket balls, minie balls, and some other relics of the past.

When he got home, he cleaned the button up a little so he could see what was imprinted on it.  On the face of the button he could make out the words, "En Union Y Libertad" circling the top of the button and an image that could barely be made out.  

The button is smaller than a dime.  The back of the button is concave with what appears to have been a loop-style shank that had been broken off.  A small amount of the gilding can be seen but most has already flaked off.

Front of Button

Magnified Front of Button

Magnified Rear of Button

After a little internet research he found a bit of information on the image, lettering, and meaning behind it.

En Union Y Libertad translates to "In Union & Liberty" and was an expression used to symbolize the independence of the Central American colonies from the Spanish during the colonial revolution in the early 1800's.

The image on the reverse side is the seal of the General Constituent Assembly of 1813.  It is the origin of Argentina's current coat of arms, or Escudo Nacional. The Assembly officially approved the seal on March 12, 1813. 

• Two shaking hands: In the center, these hands represent the unity of Argentina's provinces.
• Pike: The hands hold a pike together, which symbolizes power and the willingness to defend freedom.
• Phrygian cap: The cap sits on top of the spear.
  

It is pretty clear that this button is some how linked to this movement in the 1800's but we have been unable to find any other images of it being used on a button. Only on coins like the one in the image below.  How could such a button from this period end up in the middle of the United States?

Coat of Arms on 1813 8 Reales