Cool find! I think that Panama has used a fully dollarized currency for many years. The Balboa is pegged to the dollar and they keep enough dollars on hand to back the peg. The subsidiary coins issued by Panama generally have the same size, weight and value as the comparable US coins. So I guess it makes sense that Panamanian coins would mix in easily with US ones. Here's a denomination we don't have in the US, though: 1¼ Centésimos 1940
Wow! I've seen the 2-1/2 cent "Panama Pill" coins, but I never knew they had a coin that was half of that value! Wonder how many arguments arose over getting a cent rather than a cent and a quarter in change...