The Reagan inaugural committee, apparently afraid that some congressmen might show up in white socks, has issued an official congressional dress code for the president-elect's big day.

Congressmen, who sometimes show up at events wearing the darnedest things, are being told in no uncertain terms what to wear from head (only four-in-hand gray stripped ties, please) to toe (black oxford shoes and socks).

The word went out in a "Dear Colleague" letter this week from Sen. Mark O. Hatfield (R-Ore.), chairman of the Joint Committee on Inaugural Ceremonies. The first page of the letter lets readers know when (11:30 a.m. Jan. 20), where (the West Front of the Capitol) and why (Ronald Reagan's swearing-in ceremonies) they are to gather.

Then, there is the politically touchy business of tickets. Senators get 28 seat tickets and 200 standing tickets. Representatives get 21 seat tickets and 95 standing ones.

The letter gets tough on page two, where it says: "President-elect Reagan has suggested that participants at the inaugural ceremony, including members of Congress, wear morning attire to the swearing-in."

To make sure that everyone knows what he is talking about, Hatfield lists "a description of the appropriate dress for men which is available at numerous formal wear shops in the Washington area."

It includes "semiformal oxford gray stroller jacket (club coat), gray striped trousers, gray vest, four-in-hand gray striped tie, white shirt (plain or pleated with studs) [and] black oxford shoes and socks."

There's not one word of advice for women; presumably, they know what to wear without such help.

Asked about the letter and the Reagan dress code, transition press spokesman Jim Brady said, "I haven't seen it, and I don't believe it."

It will cost congressmen about $35 to get decked out properly for Reagan's swearing-in, according to several formal-wear rental firms. If someone wants to buy the outfit, he may be in trouble. "Nobody makes those things anymore," said Majer Raciazek of Formal Attire Ltd. "They have another model, but, it just doesn't look the same."

There is no word on whether it would pass the dress code, either.