Try G&M Tools in Sussex. It won't be listed on their Website but they tend to have quite a few Boxford bits kicking around. They advertise on here (see right). Nick.
It would be good if anyone who has a lathe of this type could just unscrew the item and take a photo of iot so we'd know what it looks like in the event that I need to either make or, have one made.
Wow, that's the shiniest Model A I have ever seen. The part you seem to want is the barrel lock not the tailstock lock which makes a difference and the tailstock is a slightly later version than original I suspect as it isn't using the split barrel body lock method.
As that is newer than mine I don't know what the missing part dimensions are however someone might be able to post a picture and you should research 'split cotter shaft lock' to find out the principle of the part you need to make.
I think there is a bit more to it. The working parts are 397 and 398 forming a split cotter. They might still be in there but I suspect they went missing when it was stripped for painting.
If nobody else comes up with details I may be able to provide photo and dimensions tomorrow evening. In the meantime you need to measure the hole and buy some brass about that size or bigger.
I suggest you also join the Yahoo Boxford group and your local ME club.
It looks like a standard Boxford tailstock, if so the parts you need are easily made.
1. a 2¼" length of 5/16" steel rod (round bar), threaded 5/16" BSW at each end, one end threaded for 5/8", the other for 7/8". The shorter thread screws into the bottom of the hole in the casting.
2. a 3/4" length of 5/8" dia phosphor bronze or brass, drilled through clearance for the threaded rod, with a corner cut off.
3. a ball handle tapped 5/16" BSW, or a nut and spanner.
see below. The rod is screwed into the base of the hole in the casting, the PB/Brass piece (sliding cotter) slides over it with the cutaway down and inwards and the handle/nut presses it down. There is a "window" inside the casting into which the barrel protrudes and the sliding cotter presses on the barrel through it. There is only one sliding cotter, the form for the second cotter is machined inside the casting
Do as Boxford do and ignore the prices on the Boxford website. I wanted a tailstock eccentric shaft – it was listed about £35 on their website at the time, only last year I think it was. But when I made direct contact the price rose to £133 + delivery + VAT.