Sorry Yoong Liat, I think I didn't made it clear. The abbreviation of PS was made up arbitrarily. I didn't mean postscript. I could have used anything like BS, CS, DS, etc. in my question.
Regarding the apostrophe for abbreviations, let me quote some of the references I've read:
The Columbia Guide to Standard American Englishhttp://www.bartleby.com/68/50/4650.html
usually add -s: two Xs, Ph.D.s, MIAs, 1990s, the ’20s. Use an apostrophe only when you need it to prevent confusion: Mississippi has four i’s. He got A’s in both courses.The Economist Style Guidehttp://www.economist.com/research/styleGuide/index.cfm?page=673905
Thus IOUs, MPs' salaries, SDRs, etc.Times Online Style Guidehttp://www.timesonline.co.uk/tol/tools_and_services/specials/style_guide/article986718.ece
An apostrophe should be used to indicate the plural of single letters - p's and q's.(Note that it doesn't say an apostrophe should be used for plurals of abbreviations.)
Guide to Grammar and Style by Jack Lynch
http://andromeda.rutgers.edu/~jlynch/Writing/a.html
Apostrophes are sometimes used to make acronyms or other abbreviations plural (another matter of a local house style). My preference: don't use apostrophes to make abbreviations plural — not "They took their SAT's," but "They took their SATs." The only exception is when having no apostrophe might be confusing: "Two As" is ambiguous (it might be read as the word as); make it "Two A's."Back to my original question. What I would like to ask is whether I should use "s" or "es", i.e. the choice of
1a. There are two s'es in the sentence.
1b. There are two PSes in the meeting.
OR
2a. There are two s's in the sentence.
2b. There are two PSs in the meeting.
In addition, how should they be pronounced?
PS I have never seen P.SS. or p.ss. My copy of Random House Webster's Unabridged Dictionary does not have such an entry nor does any other dictionary I checked.