Yes it should be not vented but it seems odd that it wouldn't say so like most of these examples my friend has in his 65 parts collection.
He was asking me why one is plain and I said maybe it's some Canadian car attribute since mine is that way.
Odds are against it being original though with how many chances there are for it to be lost over the span of 55 years.
His examples all appear to be Stant brand so maybe embossing them "not vented " is a running change, but then again the difference could indicate a replacement many years newer?
-- Edited by Oshawa65SS on Sunday 16th of January 2022 02:12:33 PM