In the book-printing world, "P.M." in small caps is the preferred representation. Chicago's specific phrasing is "usually set in small caps" (8.48, 14th Ed.), but in the commentary and examples, small caps is the only form demonstrated or specifically referred to.
The Allyn & Bacon Handbook (typical U.S. college handbook) is more encompassing of non-typeset productions and states that
Clock time, indicated as prior to noon or after, uses abbreviations in capitals or lowercase (emphasis mine). [As in] 5:44 P.M. (or p.m.) / 5:44 A.M. (or a.m.)
When typeset, A.M./P.M. often appear in a smaller type size as capital letters: [example omitted].
So, "2 p.m." is definitely the most correct form with the least intrusive modification for the primary example given.
In most cases, numbers/amounts are separated from their units with a space. This makes me question "18Z," which I take to be Zulu time, but I haven't looked into that aspect of it.
As you can see the difference in point of view when the Chicago guidelines are compared against the Allyn & Bacon guidelines, I think that we must additionally realize that the electronic-publishing world has its own limitations and accepted practices, just as the world of typewriters and photocopiers does compared to the production of typeset books. Therefore, when confronting "time stamps" for computer-generated data, there is little or no reason to be fussy about the missing periods as in "Last Updated: 07:13 PM GMT."
In a world where we are clearly trending towards minimalism in punctuation, it would be hard to say that omitting the periods for A.M. and P.M. even in running text is necessarily "wrong." As always, consistency is an important key in establishing credibility. Nevertheless, where the standards of formalism tend more toward valuing traditional and conservative approaches, then periods should be employed.
In the continuum from "old school" to "new school" I would place the options for running text in this order:
P.M. (small caps, if possible) --> p.m. --> PM --> pm
Personally, I do not see any cause for accepting the lower-case presentation without periods ("pm") in formal writing (not withstanding the fact that apparently the Wikipedia is prepared to tolerate it). It seems to me to violate expectations about how we typically deal with the abbreviations of units that are not punctuated, like L, Mbps, MB, and so on.