Spindle size for Jet diving head

I picked up a Jet diving head. The only marking on it is 74178 (model or serial # ?). I want to make a backplate to mount a 4 jaw chuck. The spindle measures 1" 3/16 - 8 TPI. The thread guage seems to be just a hair off. Is it possible that this spindle is a metric size?

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jim n judy
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