timeone
Yes, the arrow in your picture is pointing to the pin which selects the back gear or main train.
In normal turning the back gear level is push back towards the belt pully and the front pin lever should be to the right hole. If you then put the pin lever to the left this disengages the main gear train and the chuck becomes totally free turning and disengaged (with the back lever still in the back position). If you then pull the back gear lever to the forward (front of machine) position with the pin lever to the left still this engages the back gear. If you then move the pin lever to the right with the back gear lever forward, the chuck totally locks up - my understanding is because you`ve selected the back gear and main train at the same time but I might be wrong about that. However that is certainly the combination on my boxford.
Now it is interesting, after writing all that I`ve just looked back at your picture and noticed your pin select lever is to the left which would either be totally disangaging the drive or engaging the back gear - so it looks like your could be the reverse of how I`ve described it unless you`ve moved it to see what it does before posting the pic!!
If you google "Boxford Know your Lathe pdf" some kind sole has put up a copy on the web

Let me know how it goes.