About the size of 5C.
May have been made for one particular multi part job, one never knows, I have a drawer full of tools that I made 20-30 years ago that I haven't a clue what they are for, memory fades over time. I now toss such tools in the bin if I do not have the good sense to mark them.
Which I do not for the most part (-:
This was when I worked for myself, at the small shop where I currently work the Owner insists that when I make a tool or fixture that one of the mill operators engrave it with a number, I do this about 1/2 of the time, pisses him off to no end.