This video explains the differences between the Rochester 2G and the Mercarb. These carbs are often confused as they look nearly identical on the outside. Couple of things to keep in mind:

• If your marine carb is from the late 80s or 90s, it’s most likely a Mercarb
• If it has one idle mixture screw it’s a Mercarb. Rochester carbs all have two
• Mercarbs all have the carburetor number stamped into the bowl just below the fuel inlet
• Mercarbs are typically painted, Rochester carbs generally are not

https://youtu.be/yES-ZsePGIA