With reference to the letter from Ann Slater "Put full stop to apostrophe misuse" (June 15) - I reached for my tattered copy of Current English Usage - A Concise Dictionary by Frederick T Wood, price 6s net, published by Macmillann 1963.
I purchased my copy then when embarking on my GCE exams at grammar school. It has been my faithful reference guide since, through A-levels, teacher training, over two decades of teaching English, and an MA.
Under the heading "apostrophe", Mr Wood explains its uses: to indicate omission of one or more letters from a word; to indicate the genitive case; "for the plural of words which, not being nouns or pronouns, do not normally have a plural form (if's, and's, and but's. He gets mixed up with his will's and shall's); for the plural of letters of the alphabet (Mind your p's and q's. How many l's are there in travelling?); and for the plurals of numbers when written as figures (He makes his 8's like3's. The 1930's). But if a number is written as a word, no apostrophe is used: "A woman in her fifties. Things are at sixes and sevens."
I appreciate that English usage changes over time, and I have found advice in an internet guide which tells me this use of the apostrophe is absolutely incorrect. However, I do not recall there ever being a law passed to say this is the case, and I'm afraid to my eye PC's looks correct, as does 1,000's of uses. I think I would also have written Tipp-Ex'd.