Last week, Gary Imhoff proposed in Free for All that the frequent letters protesting the content of various comic strips all be designated ''Letter #1,'' followed by the name of the comic strip and the offense at issue. Taking his proposal a step further, the following additional themes could be classified by a letter number:

Letter #2 would be for all grammarians who are horrified that The Post allows writers, and even editors, to let grammatical errors of real or imagined significance defile the content of an article. Usually such letters are from English teachers who, after describing the proper grammatical form in excruciating detail, proceed to claim that their students would be offended by such errors. That is, if they ever read any part of The Post beyond ''Letter #1."

Letter #3 would be for all of those trivia experts who, like the grammarians writing Letter #2, offer tiny corrections of factual matters in Post articles or describe the omission of some obscure fact that obviously should have been covered.

Letter #4 could be reserved for the writer offended that The Post did not recognize the anniversary of some historical event, no matter how obscure, or cover a current event no matter how minor or, the greatest slight of all, did cover the event, but in the Style section instead of on page A-1.

Letter #5 could be dedicated to all the letters in Free for All protesting the quality of The Post's delivery service.

Using the above procedure, the six letters in the first column of last week's Free for All could be condensed as follows:

Letter #3, ''Aptitude Test Tests,'' median, not mean.

Letter #5, ''Post delivery,'' wet.

Letter #2, ''Grammar,'' lie, not lay.

Letter #4, ''December 7, 1941,'' Pearl Harbor.

Letter #3, ''Senate,'' approves, not ratifies.

Letter #3, ''Chelmsford,'' town, not suburb.

-- Bob Phillips