This is a modification of the terrific Antweight Robotwars Firestorm R/C Robot (see remix link)
The original arm didn't fit my servo, which I think is pretty standard. The body has also been redone to hold n20 drive motors, kept in place with small zip-ties and a more standard servo mount set-up.
The other mod from the original are more robust hinges for the flipper. Still fairly fragile, at least there's 2x the meat holding on there. The down-side is a less flush flipper (no difference in performance!), at the point of the hinges. A small price to pay.