So I got Kripke's 'Naming and Necessity' via interlibrary loan, thanks
to having been invited to that other short-lived list.

I'm reading how 'Nixon' is a rigid designator as it means "this guy"
or "him" across all possible worlds in which "he" should appear.

The "possible worlds" lingo is somewhat parochial to philosophy, with
fiction's "believable worlds" a close relative. To project Nixon into
a believable world is to take advantage of any associative powers this
word has in the reader, and to use these to build up impressions
(helps add color and texture to a blank canvas). In this sense,
proper names are like tools in one's palette, designed to leverage
prior reader experience.

There's also the language game of personal responsibility to consider.
It's essential that we have this "could have been otherwise" tense,
i.e. choices matter and outcomes might have been otherwise. Nixon
might have lost the election. Our sense of justice, accountability,
punishability, hinges on knowing that someone did A, but might have
done B. And in that sense, we develop this image of "branching
worlds" i.e. a possible world is one in which someone did, in fact, do
B instead of A.

Kripke's own "possible world" concept is rooted in this notion of
"branching". A possible or believable world that's simply parallel,
but at no point "branches off" our own timeline, would be more
problematic, especially if a *lot* of parameters were tweaked. We
might imagine a person who's the spitting image of our Nixon (a fairly
unforgettable visage, to those who've seen him), but who works as a
brain surgeon in some city. But the landmasses are different --
there's no "North America" per se, no English speakers. So we'd be
using the word 'Nixon' as a shorthand for the uncanny likeness in
appearance (uncanny in the sense that we're imagining it'd be highly
unlikely for a world so contrary to ours, to still contain someone
who's the spitting image of Nixon). Likewise, a face on Mars (pile of
rocks, seen from a certain angle), might be called "Nixon's face" (for
the benefit of the reader -- characters in this other world might call
it something else, or nothing at all).

This *sounds* like we're actually viewing a parallel universe "through
a telescope" whereas, as Kripke points out, these parallel worlds are
defined not through observation, but through stipulation. He's
wondering what it can mean to *stipulate* Nixon in a parallel
universe. I'm saying there's no one proper sense in which this
stipulation might be made. The connection could be very distant
(between the stipulated world and ours), and 'Nixon' might simply draw
attention to a facial likeness, minus any shared personality traits or
professional history. On the other hand, we could write a movie
script in which a character very like Nixon in persona (more than in
physical likeness), participates in rather different scenarios.

In the latter case, the premise might be "Nixon starts WWIII" or
"Nixon tells the truth a little more often" or "Nixon, if Kissinger
hadn't lived" -- all parallel worlds, with or without a branch point
spelled out. Most people would likely "buy" this use of Nixon,
because we have lots of precedent for historical fiction, fiction
involving characters drawn from real life, plus a strong sense of "our
world had different actions been taken" (linked to concepts of free
will, volition, contingency and so on). In the former case (physical
likeness, nothing else the same), people would be more inclined to say
a "Nixon-clone" or "guy who looks just like Nixon", but without
agreeing to equate or identify him with our "real world" Nixon. But
there's a whole spectrum of examples between these two options, and
you could expect strong divergence of opinion on whether calling this
alternative world character 'Nixon' were apropos. I'd say there's no
cut and dried "right answer".

In sum, our language game of "possible worlds" is not a highly
developed one (it's used somewhat sparingly -- we might imagine a
world in which we've built it up quite a bit more), and owes its
existence to related practices of fiction (imagining/fantasizing about
alternative realities) and the imagination in general (imagining
anything contrary to fact). But if we push our stipulations too hard
(e.g. "a world in which Nixon..."), we'll easy come to a breaking
point or threshold, in the sense that strong agreement breaks down,
and people don't feel we have any roadmap or obvious rules to follow.
Philosophers may try to pioneer in this territory, but I'm thinking
the writers of fiction probably have the edge, given their greater
experience at actually creating worlds with texture (philosophers tend
to make do with impoverished notions supplemented by cryptic
expressions in modal logic).

Kirby