Radiator drain plug HELP!!!

[wldcat10]

Took my ‘70 out today around to the gas station to fill her up and air up tires. As I waited for air compressor to be freed up, I got out and popped the hood to look for any leaks and boom! My drain plug blows out. I went to autozone around corner to grab a drain plug, they give me a 1/8 plug, too small. I go return it and the 3/8 is a special order item. So I grab a 1/4 drain plug and it’s too big! I looked up the size/specs and shows that 1/4 drain plug should be the one. Am I missing something? Any input is appreciated....