Join Date: Dec 2011
Location: Morris County, New Joisey
That sure seems like an odd sized nut. From the spec, "15.875 2.0", I'm guessing that it's a 5/8" OD, which makes sense as Woodnthings mentioned. (15.875 mm is 5/8", based on the accepted standard of 25.4 mm per inch.) In metric thread terminology, the 2.0 would be the thread pitch in mm, i.e., 2mm from thread to thread, which is nowhere close to the SAE standard 5/8" acme thread pitch of 8 TPI. A 2mm pitch translates to 12.7 TPI.
Metric "acme" threads, (which seem to be called trapezoidal), in addition to having a different angle in the "valleys" of the threads, appear to be available in 16 mm thread pitches of 3 or 4 mm; not the 2 mm your spec appears to indicate.
I'm guessing that the nut for your saw was custom manufactured for that saw, & possibly others, and may not be available from standard hardware suppliers.
Have you measured the threads to make sure the pitch is 2 mm?