Spanish Coins on US Colonial Notes

A few colonial-era notes depict a Spanish coin.  There is only one in this exhibit, reflecting a collecting decision not to seek such colonial notes.  For more information, see Eric P. Newman's The Early Paper Money of America. 

The note shown is from Virginia, part of the October 20, 1777 issue.  Even though it is after the Declaration of Independence, it is often considered colonial because Newman's book goes through the end of the eighteeenth century.  Another Virginia issue dated May 6, 1776 has a $5 note with a Spanish coin.  That's a legitimate colonial issue.  Why didn't I buy that one?