The Carter Administration is putting the final touches on a tough plan to drastically alter the system used to set federal blue-collar salaries and trim most of the built-in growth in the annual $6.4 billion payroll.

Biggest casualty would be the five differential steps that now guarantee, the government says, that a government employee who remains in the same job for six years a salary that is 12 per cent more than the going rate in local industry.

Under the plan, most of the 511,000 federal mechanics, carpenters, technicians and skilled and unskilled "wage would" employees also would keep some or all of the valuable pay-differential steps. Tha would affect not only-future salary, but also the value of life insurance, sick and annual leave and pensions, which are based on maximum salary.

There are 30,000 blue-collar civil servants here. They are paid under a system separate and distinct from the 300,000 white-collar government employees, or the postal service's 12,000 workers here. Blue-collar wages are tied to local industry rates for similar jobs. White-collar employees get national, rather than local pay rates and postal wages are set by union contract.

President Carter has not given a final okay to the blue-collar salary shakeup plan (tougher than the proposal President Ford had drafted) that his advisers have worked out. But insiders believe that he will go with it and federal unions are already lining up congressional friends to block any significant pay changes.

The White House package would drop the legal requirement that all blue-collar workers get a five-step pay differential in each grade. It also would eliminate night differential rates for many blue-collar workers, leaving them only for government employees in areas where night differential is the prevailing practice in local industry.

The plan also would eliminate the so-called Monroney amendment. It requires government salary surveyors to go outside a local wage area - like Washington, or Atlanta or Dallas - to get wage data if the local private employer doesn't have similar jobs for pay comparison.

On Feb. 28 this column reported that the White House was considering a plan to drop some - but not all - of the live-step, differentials. Officials say that pieutre has changed. The plan now is to eliminate the automatic guarantee of five differential steps for all employee. Instead, they say, differentials would be based on prevailing industry practice.

If the common local industry practice (as determined by government) was for a two-step pay system, the government would adopt it in that area. If it was a five-step system in industry, the government would use that. Nobody would be guaranteed five steps, or any number other than the prevailing local practice.

Government unions will view any plan to cut back benefits - promised them by law by earlier Presidents and Congresses - as the easy, but unfair, way to trim salary costs. They will point out that since the government controls local wage surveys, every effort would be made to "cheapen" the local private industry pay picture so as to give the government an excuse to pay its blue-collar workers less.

What Distress Call? When Health, Education and Welfare was under fire last month for some interesting job descriptions, some employees noted that the departmental flag outside headquarters was flying upside down. They took it as a "Mayday" call, since an upturned flag is a signal of distress.

Not so, say officials. It was all an honest mistake (the flag looks like it is upside down even when it is rightside up) and nobody was calling for help. Be that as it may, flag attendants now are very, very carefully watching to make sure the HEW standard goes up the flag pole right-side-up.

Or Was It A Mistake? Some career workers of the elite HEW Secretariat office have been told - rather rudely and bluntly they feel - they aren't wanted. Nobody is to be fired, but about a dozen of the troops in the 100-person decision-filtering office have been told to find other jobs in HEW.

Some of the unlucky dozen say they were given 10 days to find other perches and one claims he got a follow-up call from an official two days after his eviction noticee to inquire when his desk would be vacated.

HEW officials say they are unaware of any such pressure. They agree that some workers have been told they would be better off elsewhere in the department. Secretary Joseph Califano has ordered the secretariat pared down from the move-on orders are not - officials say - dismissals but rather transfer actions.