I show a brass and the lighter bi-metal... only not the same side. Some of mine are slightly different in color but I haven't seen any major consistencies in this so I assume what I have is some fading or light/air/ oil/ dirt/ wear deviations.

No micrometer needed... but gem scale would help!

http://www.geocities.com/mschips_az/BR01.html

Did you know the only way you can tell the difference between a 1982 Zinc and Copper penny is by weight?