Richard Crowley wrote:
"Cecil Moore" wrote ...
From Webster's: "apostrophe - a make used to indicate ... the
plural of letters or figures."

To take the argument well into absurdity (as if it
needed any help), we can discuss whether your
Webster's is descriptive or prescriptive.

That definition agrees with what I learned in English
class 50+ years ago. The plural of a number uses an
apostrophe. So 73's would indeed be the plural of 73
as in "many best regards". It's no stretch to assume
that the plural of SWR would be SWR's.

Incidentally, I have an obvious typo in the definition
that the spell-checker didn't catch. Should have been:

"apostrophe - a mark used to indicate ... the plural of
letters or figures."

I have a "Webster's New Universal Unabridged Dictionary"
which contains a "Basic Manual of Style" in the back.
Here's what it says for "Apostrophe: ... 3. To form the
plurals of letters or figures add an apostrophe and an s."
That's seems to be prescriptive.
--
73, Cecil http://www.qsl.net/w5dxp