Ttrees is correct.....Startrite used imperial threads on their earlier models and metric on the later models, but there was a transitional period where they used both on the same machines apparently.....!
I cant say what the metric thread would be, but if its an imperial thread, 5/16 UNC fits because I've replaced a few on a couple of Startrite table saws. I've worked on 4 different saws and so far they have all been imperial threads, but all 4 saws had an Imperial rail.....