There have been two versions of the dual VNH5019 motor driver shield. The revision identifier, ash02a or ash02b, can be found printed near the upper left corner of the board (below the mounting hole). While either version should generally be a drop-in replacement for the other, there are some minor differences between the two versions.
The first revision, released in October 2011, was ash02a.
This version predated the Arduino Uno R3, so it lacked pass-throughs for the four new pins present on the R3 and all newer Arduinos.
Schematic diagram for revision ash02a (87k pdf)
The second and latest revision, released in March 2014, is ash02b.
Compared to ash02a, the following changes were made:
Schematic diagram for revision ash02b (356k pdf)