There is not a mast step in the boat. probably not a yoke, little need to portage in the swamp or marsh. It is not needed as a thwart (structural support), because the boat is strong enough as is. I'm thinking it relates to trapping but made to be removed when not trapping.
If that was a cradle to hold a punt gun it doesn't look like it would be very secure and there is no cradle in the bow to keep the barrel
secure in one spot.
We might keep in mind that what we are looking at may only be whats left of whatever it was. A puzzle with parts missing. Just a thought.
I doubt if it is for a punt gun, but it is the only picture I have found that looks similar. The boat is too small for that activity. It is not an old boat so I think the piece was built for a purpose and would be complete.
In the middle of the Okefenokee Swamp there is an Island ( Floyd's Island ) and an old hunting lodge where you can camp and it is a 1/2 mile portage across it , either way , especially if you want to complete your trip across the swamp.
The only time I had to do a real portage in a swamp besides just using the bank to get around some blow downs.