TAKE A WOMAN in her 80s who has very little money. One day soon she will open an official-looking envelope containing a machine-typed letter that informs her that the Social Security check she's been receiving for over 15 years will henceforth be reduced by about $100 a month, more than a fourth of her poverty-level income. There's some fine print she can't make out. It says that, if she'll dispose of her modest savings, she might be able to go on welfare to make up some of her loss.

Tear-jerker? Of course -- but it's also a live possibility for many recipients of the Social Security minimum benefit who, under the provisions of the two budget bills now headed for conference, will share an income loss of about $1 million starting next year. We don't know how many such cases there are among the total, but neither does Congress or David Stockman. Perhaps this is why they found it easy to make this cut while shying away from others affecting more vocal segments of the Social Security population.

The Social Security Administration estimates that there are about 3 millon people receiving benefits close to the minimum, 2 million of whom will be likely to face a significant income loss. These are people who, as a result of low wages or relatively few years of employment covered by Social Security, would qualify at retirement for benefits smaller than $122 a month. Some may have worked for many years as domestics, farm workers or in low-wage self-employment not covered by Social Security at the time. Others may have entered the labor force late in life after the death of a supporting relative or desertion by a spouse.

On the other hand, some with substantial property income or a working spouse may be relatively well off. A small percent are retired federal workers, some with ample pensions, who earned a Social Security benefit in a few years of private sector employment. The only estimates people have on how many fall into what category of need come from a 1977 survey, but since it only covered people then entering the rolls it didn't give a good picture of minimum beneficiaries, most of whom are known to be quite old -- over half a million are said to be in their 80s -- and almost all of whom are women. A third of the group might recoup some or all of their loss from welfare, if they're willing to apply. Welfare grants, however, are typically less than a poverty level income and are offset by income from savings or pensions while Social Security is not.

Federal retirees are cited as the main target of the cut, although they make up only six percent of the group and the better-off federal pensioners who get more than the minimum won't be affected. If others in the group are similarly well-situated, as the administration claims then surely nothing can be lost by adding a little safety net. How about exempting from the cut anyone with an income of less than, say, $5,000 a year? If no one is that poor, it won't cost anything. If, on the other hand, it turns out that many of the people at risk are both old and far from well-off, these are savings the society should reject.