IN THE LETTERS column today, Assistant Secretary of Defense Lawrence Korb objects to recent Herblock cartoons in which Defense Secretary Caspar Weinberger was pictured wearing around his neck a toilet seat with a $640 price tag attached. Mr. Korb does not express objection to the indignity of the portrayal. His concern is with the details of the drawing.

The Department of Defense, writes Mr. Korb, "did not pay and never has paid $640 for a toilet seat as portrayed in the cartoon." Despite the italics in the preceding sentence, the operative part, as they say, is the phrase: "as portrayed in the cartoon."

In fact, the Pentagon did pay $640.09 for each of 54 "seats, water closet" -- as they are described in Navy specifications -- purchased in 1983 from the Lockheed Corp. and more than $400 apiece for 70 or so bought in 1979. But while these objects incorporated the familiar oval hole, the shape of their frames differed from the standard toilet seat because they were custom designed to fit lavatories in Navy P-3 Orion planes.

Mr. Korb points out that when you custom design toilet covers and produce them in small amounts, each copy costs more than would a standard seat. That's especially true if the company producing them is accustomed to adding large overhead markups to its products before computing its not ungenerous 13.4 percent profit margin. Sloppy company record- keeping can also drive up costs. Right after the high- priced toilet seat came to the Senate's attention, Lockheed reported that it had reviewed its records and discovered that the correct charge should have been a mere $554.78 -- an amount the company later reduced to a "policy price" of $100.

Mr. Korb, however, does not address the basic issue of whether the Pentagon's penchant for custom-designed products really serves basic national security interests or is simply a bad habit that military procurement specialists have acquired from years of undisciplined buying. Nor does he explain why Sen. William Roth was told by a local contractor that even the specially designed item could be produced for about $150.

As Mr. Korb notes, when the $640 price tag was revealed, the Pentagon was in the process of asking for competitive bids on further purchases. But Mr. Korb doesn't point out that the matter came to light only because one potential bidder -- an experienced Pentagon supplier in Missouri -- was trying unsuccessfully to get copies of the bid specifications from the Pentagon and finally had to appeal to Sens. John Danforth and Thomas Eagleton for help. Competitive bidding doesn't work very well if no one but the original producer knows the details of what is to be produced.