Solved the problem another way.
Rod Peterson--Ormond Beach, FL
I didn't think there would be a gusher of response to my query, given the proliferation of lunchbox planers over the years. Plus, I've been agonizing over this thing, collectively, for a couple of years. So, yesterday, I plunged in head first.
The existing holes seemed more metric-like than imperial, so I bought a couple of samples of metric bolts for reference—namely, 5mm and 6mm. I already had some 4s and they were too small. The testing showed 5mm to be a better fit, and with the thickness of the sheet metal hood (about 10 gauge), would afford me more thread contact. So, off I went to buy a tap.
Actually, I bought both a 5mm and a 6mm tap because I wasn't fully committed to the 5mm, yet. As an aside, the correct drill size for the 5mm is #19 and the correct drill size for the 6mm is 5mm. I have number size drills—I don't have (and couldn't find) any metric drill bits.
Back home, I triple checked the apparent hole size and chucked up the #19 drill bit and cleared out the holes. I never was able to confirm whether it had been threaded fasteners previously employed or sheet metal fasteners. I chucked up the tap and threaded both holes.
I had a couple of washers on hand that would serve the purpose beautifully in spreading the pressure over the slots in the plastic dust hood (actually, much like metric fender washers, if you will), and in a couple of minutes the screws and hood were securely, er, secured.
In the final bit of happy coincidence, the 5/16" nut driver I have secured nearby on a neo something or other rare earth magnet is a perfect fit on the 5mm bolts.
Project completed—only two or three hundred to go...
Rod