The 1/2″ precision bores on the Tosa Tool PH (Pre Hard) Modular Fixture plates are machined to 0.5005″ +.0004 -.0001″.  We have found this to be the ideal tolerance when using a mix of dowel pins, gauge pins and accessories as most pull dowel pins are .5003 +/-.0002.  Keep in mind, for a dowel pin to have a “slip” fit (e.g. not require hand tools, vice grips, etc to remove), there must be some gap or “slop” in the fit between the pin and the hole.
The 1/2″ precision bores on the Tosa Tool MC (Medium Carbon) fixture plates are held to approximately 0.5007″ +/-0.0003″.  The additional tolerance range is due to the steel being less hard and tough than the PH.