PBK is correct! They are given by the Commander to soldiers who preform above standard but not good enough to warrent a real medal. They ae normally used to see who pays for drinks at the bar. The person with the highest Commander ie General woulld win over a LTC. here is one of mine.
Demon, here's another 504th challenge coin, and it measures 40mm, or a little over 1-1/2". Most challenge coins are approximately silver dollar size, which is 38.1 mm:
You are correct! However It really depends on the Commander. The 1st one is one I got from the big red one 1st Brigade CDR. the other is about the size of a morgan dollar. It is a great find the run about $12-$25 dollars per coin for the CDR to buy.
I bought my dad a souvenir type one from the "USS Intrepid" Museum in New York City.He served on it from '66 to '70 when it was part of the Pacific fleet off of Vietnam.