I have not had this issue until now. On the three convertor accordions we have (one Russian bayan, two Bugari) the melody bass has a C where in Stradella the G major is. When I ordered the bayan I asked them to only mark C and F on the first and third row of the melody bass so as to avoid mixing up the C melody bass on the fourth row with the C base note for the Stradella (these two are very close to each other).I'm not an advanced played, but I do use the stradella bass/counter bass in a couple of pieces where there is a very big jump in the LH and the music goes fast.
The one problem is the alignment of notes seems to be different between manufacturers which is kinda annoying!
I believe that this layout is pretty standard on 120 bass convertor accordions (with C system melody bass).