One thing I can definitely tell you.
It isn't an original fitting from your lathe. With the curved forms to the castings, they come from a much earlier or a much more expensive machine. If you look at your lathe, all the fittings that are on it are just straight sided castings, very easy to produce.
The finger sticking up I think would sit in a groove of some sorts, and looking at the same bit of casting the finger is on, there is an angled cut. So maybe this was something made up for feeding something with a slot in it, and cutting some sort of angle onto the part.
I have a box of bits just like that, most probably made to do one specific job, just the once, then thrown under the bench. You will find most modellers pick things up like that, thinking it will come in useful one day.
Bogs