I'm not sure about this, but there might be a chance it could differ depending on the year as I've seen the rails with both imperial and metric markings.
I looked through the manual but saw no writing stating the bolt sizes, but maybe it shares a bolt with another and you could check that with a regular nut.
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.....