It sure looks like a stuffing box/shaft log to me. It's even the right color
The ones on our boat use two big collar nuts, one of which is a lock nut, to adjust the pressure on the stuffing material, flax in our case. I've not seen an arrangement like yours, but then I've not seen that many stuffing boxes.
You also have a cooling/lubricating water feed from (I assume) a pickoff on your engine's raw water system. We have these on our boat, too. In our case, it's because GB used two cutless bearings in the shaft log instead of just one, and the one foremost in the tube doesn't get sufficient cooling or lube water on its own. So they pipe water from the engine's raw water system to the shaft log.
It's the reason why, in our boat, if we have to shut down an engine and proceed on one for more than a couple of miles, we have to tie off the shaft of the shut-down engine to prevent it being turned by the freewheeling prop. With no cooling water going to the shaft log (because the engine's not running to send it) it doesn't take long for the bearings to overheat pretty dramatically.
We've mounted angle plates and shackles to the engine room overhead directly above each transmission's shaft coupling to facilitate this. In the nine years we've owned the boat, we've had to use them twice, both for raw water cooling problems that required a precautionary engine shutdown.