There wasn't an R2. There's only the original and R3. It was given the R3 designation to follow the same conventions for the Uno and Mega2560 R3. The "R3" of the ethernet shield only added pin headers, which matched the "new" headers of the R3-Uno and R3-Mega. The pins added were:
1) Dedicated SCL and SDA next to AREF.
2) IOREF next to RESET and N/C next to IOREF
No actual hardware changes were implemented.