Hi John,
Here's what came on my car. i think it's correct, but maybe Loren or others can confirm. Heavy chrome plated flanged head bolt with 1/2" hex and 5/8" diameter flange. Raised "NL" in center of head (like the way the word "centerline" is usually abbreviated - L in the middle of the N). 5/16" diameter x 18 pitch thread, ~3 3/8" long. Pictures attached.
AMK has a bolt about the right size and characteristics, but no chrome plating. B-12711 on page 25 of their catalog.
There are rubber grommets that seal the underside of the flange on the bolt to the valve cover. Carmen has them; I suggest you buy a set from him to include with your carbs.
Cheers!
Chuck