I have a 12 inch Delta / Milwaukee lathe .The other day I was looking at a 6 inch faceplate that came with it .I noticed it has 2 different type of threads one was left hand and the other was right hand each thread started and ended past each center is the only way I noticed it .Because it did not have a continuous thread like a normal faceplate . Was this type of faceplate used back in the 1940s ?