I do not think the carbs are different, however, Merc has a habit of installing different jets in different carbs on their V6 models. I never checked the jet size on my Merc 1500.
It is starting to sound like the PO had some carb issues, and a unique way to try and resolve them. Getting the idle mixtures on those carbs are a bit tricky. If they are set too lean, they idle great but will not accelerate. As you richen the carbs, the idle quality gets worse, but acceleration gets better. You want the leanest that will still allow acceleration. The top carb will likely need to be set the richest, followed by the mid carb. The bottom carb will be a bit leaner than the others. Make sure to set the carbs using fresh 50::1 fuel mix.
You can switch the carbs back pretty easy. The yoke on the top carb should be interchangeable.
Choke plates should be on the top two carbs only. I added a third choke plate, and it made no difference in cold start of the motor. You can adjust the choke plates to fit tight but not bind. That does help a bit.
