I posted a couple of weeks ago that my new chainplate made to match the old one had the wrong size holes drilled in it ( made exactly the same as the old one. All the holes at the standing rigging at the mast are 9/16". The one for the backstay has 5/8ths holes. I realized the difference when I tried to purchase new 5/8th bolts.

After finding and purchasing the bolts and also getting 9/16th bolts (just in case) I found that the holes in the fiberglass are 9/16.

So,,,, it appears that the wrong size holes were drilled at the factory in 1982. Rather than drilling out the fiberglass, I'm just going to put the 9/16" ones in. If it was that way for 25 years it can do it again. Next time I'll be more observant. I just 'assumed' and you know how that goes.