Absence of the period (aka full stop) after the timestamp appears to indicate that a commenter had previously (perhaps recently) posted a comment which was subsequently flagged. Or that the commenter is new (see below).

Here's what I did:

I used ctrl-f/cmd-f on a well-commented post (https://news.ycombinator.com/item?id=25190668) looking for the following search terms: ". [–]" and "o [–]"

Then I reviewed some of the posters' recent commenting history. Those that had no period after the time-stamp had recent grayed-out comments. I did not find any commenters with a period who had recent grayed out posts.

For instance, here's a page from esteemed commenter buran77's profile: https://news.ycombinator.com/threads?id=buran77 . You'll notice that not only does buran77 have grayed out posts without the period-after-timestamp feature but so does grayed-out commenter elmo2you.

The absence of the period also appears to apply to recently created accounts. See green commenter jn6118 in this thread.

This appears to be a subtle indicator designed to be used in HN's comment moderation. If so, what can we glean from this quirk of HN's comment moderation procedures or policies?