The sixth installment of 16 stamps showing flags of countries in the United Nations is coming out Friday. It brings the series, which was started in 1980, past the halfway mark.

The new segment displays the flags of these 16 nations: Grenada, West Germany, Saudi Arabia, Mexico, Liberia, Chad, Mauritius, the Dominican Republic, Oman, Ghana, Sierra Leone, Finland, Uganda, India, the Soviet Union and Sao Tome and Principe, known to American collectors as the St. Thomas and Prince islands.

This is the first segment to bear 22-cent values. The stamps in the initial 1980 segment were 15-cent values; the following four segments were all 20-cent denominations. The flag stamps have always been released only in United States currency, with no issues in Swiss or Austrian currency for U.N. offices in Europe.

The stamps in each issue are produced in multicolor photogravure according to designs by Ole Hamann of Denmark, former head of the U.N. Postal Administration, with execution by Thomas Lee of China and Rocco J. Callari and Irving Konopiaty, both of the United States.

Any of the four full sheets of four stamps each that make up an issue may be purchased individually. Collectors who wish to buy a block of four of a particular stamp must buy the entire sheet of four blocks of four.

Single stamps are available in four sets. Each set includes the four different stamps from a sheet. Collectors wishing to buy a single of a particular stamp must buy the set of four singles from that sheet. The cost of a set of singles is 88 cents; the cost of a full sheet is $3.52. Also available from the U.N. are official first-day cacheted covers. There is a handling charge for orders under $2.

Orders or requests for information should be sent to U.N.P.A., Box 5900, Grand Central Station, New York, N.Y. 10163-5900.

There were no eye-catching flags to unfurl on the cacheted covers traditionally put out by the World Federation of United Nations Associations (WFUNA) to accompany new U.N. issues, but it has gone beyond mere national symbols to provide an attention-getting look at a surrealist cosmos.

The untitled watercolor was contributed to WFUNA by Wolfgang Hutter, a 57-year-old Austrian whose work has been acclaimed in Europe. In addition to the standard-size, first-day covers, the organization has also prepared jumbo-sized envelopes in a format large enough to accommodate a full sheet of flag stamps.

WFUNA has prepared four standard covers, each with four different flag stamps, for $8.15. Four covers with the center blocks from each sheet -- the four different center stamps joined to each other -- are $10.15. Covers with a single flag stamp on each -- 16 covers -- are $14.65. Also available are sets of blocks of four of each flag stamp and similar sets with marginal inscription blocks of four. These 16 covers are $14.65 and $27.60.

There is a postage and packing charge of 50 cents for one to five standard cacheted covers and 20 cents for each additional five. The charge for one jumbo is 80 cents, for two to five $1.20, for each additional cover 20 cents.

Lithographs of the Hutter watercolor in a limited edition hand-signed by the artist are available with no stamp affixed or with a first-day cancellation of one of the new 16 flag stamps, as designated by purchaser, for $50.

Orders or requests for more information should be addressed to WFUNA, Room DC1-1177, United Nations, New York, N.Y. 10017.

Israel has issued four commemoratives to usher in the Jewish New Year 5746, which begins tonight with Rosh Hashanah.

The stamps show the Ark of the Covenant, a box of acacia wood overlaid with gold; the table of acacia and gold that stood in the Tent of Meeting; the menorah, or seven-branched candlestick; and the altar of incense within the Tent of Meeting.