I realize this is looking for a needle in a haystack but I thought it was still worth the chance.
I'm looking for the vertical milling attachment Brown & Sharpe sold to go with the "0" and "0Y" milling machines. The illustration from the 1911 catalog is linked below.
My machine was made before this accessory was offered but I am hoping that the dimensions didn't change. The spindle should have a #7 B&S taper. The machine itself has a #9 taper.