JLLebbert - Look at the center vein of the leaf ID'd in the picture of your stamp below. Note that it is broken-up into 3 or 4 pieces. Now look at Scott 500's and the 528 series stamps on Siegel. You will see that vein is generally a single, or at most a 2 piece line on the flat plate #500 whereas all the offset varieties it is broken into pieces.
Yours:

Sc# 500
