According to the serial number chart, your machine was built sometime in 1973-1978. During this period Shopsmith built 1001-20359. Based on the serial number and interpolation I would place it in 1975-1976. These machines would be 500s.
SN 013870 = 13870
However, I can see why you might believe either 1985 (000016) or 1986 (213306)
Notice that in 1985, 1986 and 1987 there are two different starting serial #'s. I believe these are 500s and 510s which were built during that time period under two different serial number sequences. The 510s first went into production in 1985.
I didn't know that there was a 500 with a 2 bearing quill but the chart now leads me to believe there was. Serial Number 190000, built in 1984, had a two bearing quill. The 510s began in 1985
Yup, I see why you are a little confused. So am I. So is this chart.
"Making Sawdust Safely"
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