Thanks for unclogging my brain. I was putting a square peg in the round hole. As soon as I put the calipers to the hole, it was obvious. They are 0.159 inch diameter -- thus 4 mm. I don't have any suitable M4s, but a flathead M4-70 screwed right in. I know I can get panhead and cheesehead M4s in town.
Thanks all.
