Cool counterstamp but not an error. Made by some random dude with a letter/symbol punch. Not sure what the counterstamp is, maybe a fancy "D"
I have now idea what and ampersand is. Luckily I have my good old tutor Wikipedia to help me. "An ampersand is a logogram "&" representing the conjunction word "and". This symbol originated as a ligature of the letters et, Latin for "and"." Cited from http://en.wikipedia.org/wiki/Ampersand