Saving Old Seagulls

I managed to get the Mariner running on the bench after a bit of tweaking and like it so much I thought I'd see if I could find the outboard version.

Just secured one but won't be able to collect it until May - in the mean time I was wondering if anyone knows the function of what looks like a regulator plate on the exhaust expansion box (pic attached). I assume that the exhaust mainly goes down the leg - is this device to facilitate starting I wonder?

Thanks,
Bernie

Yes, that rotating plate is there to assist starting, by reducing exhaust back pressure.

Quite an interesting bit of engineering for its day the BMB outboard leg. first used in the early 1930s.