For the 621 residents of the town of Quantico, the acquisition of a tiny strip of land along the Potomac River yesterday was a giant victory.

"This is, I think, the greatest day in the history of Quantico," said Town Council member Al Brown.

The reason for the jubilation: The 40-acre Prince William town, surrounded by the enormous Quantico Marine Base, is no longer landlocked. At a 10 a.m. ceremony yesterday, the four-acre river site was officially transferred from the federal government to the town of Quantico.

"This came as quite a revelation to me, because I always thought that Quantico was on the Potomac," said Rep. Stan Parris (R-Va.) of the 8th Congressional District, who town officials said was instrumental in arranging the transfer. "Well, until today, it was close to the Potomac, but not on it."

After more than a year of haggling, the U.S. Marine Corps agreed this year to classify the plot, which was purchased by the corps in 1942 from private owners, as surplus land, thus opening the way for the transfer, Parris said in an interview after the ceremony.

The land, which was given to the town for free, will now become Quantico Municipal Park, where town officials hope eventually to build a picnic pavilion and a marina.

"Many Quantico residents do not have access to a lot of the Marine Corps recreational facilities . . . . This will be the town's own," said Mayor Lively C. Abel.

But those same Quantico residents have something besides recreation on their minds. Officials hope a marina will draw boaters off the Potomac into Quantico to spend money in its stores and restaurants.

Quantico's economy has suffered since the formerly isolated Marines gained easy access to Washington in recent years by the increase in cars brought to the base and the construction of Rte. I-95.

"I first came here as a 17-year-old private in 1942," said Prince William County Supervisor Edwin C. King, whose Dumfries District includes Quantico. "Back then, you didn't get paid much, but what you did get you spent in Quantico."

Quantico cannot afford to build the park entirely with its own revenues, and will seek federal and state grants, Abel said. Yesterday, King presented Abel with a $7,500 check that had been allocated to the town last week by the Prince William County Board of Supervisors to spend on the park.

Even if Quantico's dreams of economic resurgence are not realized, yesterday's acquisition will finally restore the logic to the town's name. "I am told the word 'Quantico' comes from the Indian word meaning 'by the large stream,' " said Parris. "Until today, that name was a misnomer."