Most of the online stores are selling the wrong M switch and a good many the reg as well. They don't do their homework and think all Dubuques are the same.
As long as you used the 420 with a regulator diagram the only difference you'll see is the regulator will keep it from overcharging the battery on long runs. The cut-out won't regulate the current. The 420 style is better even if it isn't correct.
At 6.6v you are charging but I doubt that would move the needle. It usually doesn't on a fully charged system except at start up. If you aren't staying charged double check your grounds and then go through John T's troubleshooting guide. You'll find it here easy doing a search.