Its is a Drive Plate for a lathe. It is a D1-4 spindle mount so it will fit any lathe with a D1-4 camlock spindle. It is universal.

Its is a Drive Plate for a lathe. It is a D1-4 spindle mount so it will fit any lathe with a D1-4 camlock spindle. It is universal.

References:

https://www.machineryhouse.com.au/L310

D1-4 camlock adapter for a metal lathe. It has a plate bolted to the front of the adapter that may have been to drive a shaft via a "lathe dog" that engaged either the single pin or the allen-headed bolt. I've included a link to the definition of a D1-4 spindle, and a wikipedia photo of what a lathe dog is and how it engages the spindle plate to drive a workpiece. The key to the spindle adapter size is in the 3rd photo in your set of 5 photos where you are measuring the tapered hole in the center of the 3 attachment pins. It shows as roughly 2.5 inches and if you go to my "specs" link you can find the large end of the taper is 2.5005 diameter, (dimension "B") You can also check the attachment pins at 5/8" diameter (dim. "F")

References:

http://www.tools-n-gizmos.com/specs/Lathe_Spindle_Mount.html

https://en.wikipedia.org/wiki/Lathe_dog

Reason:
Another opinion was accepted instead.