I know the spindle thread on my SB is 2 1/4-8. I have a few odd back plates in my extras bin. Two scale to be 2 1/4-6 and the other 1 1/2-8. I found the 1 1/2-8 could fit an Atlas or Logan lathe. I found nothing on the ones that seem to be 2 1/4-6, can anyone tell me what these might fit?