There's no such thing as an "absolute" with these trucks. The best documented source will come from your interpretation of everything you read and are told, so it's a bit unlikely you'll ever be "absolutely" sure. ESPECIALLY for a 1955 with a mid-year body style change AND the V8 option coming into play. Lots of mixing & matching and use of legacy (54-55.1) parts.
I know what you're asking, and I think you could eliminate a whole lot of guessing if you'd post the engine serial number and the head casting number. We could for sure determine the range of time the engine was built (I think even the month?). Whether or not it was built & immediately installed in your truck is a whole other story.
The three 100% original 1955 3100 2nd series trucks I've grown up with (I only own one) all did NOT have vented valve covers.
Undocumented source, of course.
Brad
Edit- Retract some of the above, because there are some peculiar configurations & years that did have vented valve covers;
Tim - tclederman has a lot of this info on his website.
