Sorry, but I don't think I have ever seen so much worry and hand wringing over the discovery of size and purchase of one bolt. Go to a decent hardware that has a good selection of individual bolts, buy one of different sizes and thread count both in Metric and SAE that you think is close to the correct diameter
( can't be more than about 6 or 7 ) Maybe $3 to $4, take them home and see which one fits. AIN'T ALL THAT HARD.