The Red gun crews, working in camouflaged pits so deep that only a direct hit could knock them out, carried on a battle of attrition that observers believe may be a forerunner of a drive into the strategic northern tier of South Vietnam’s provinces by as many as 35,-000 North Vietnamese regulars.Three enemy divisions are reported based in and around the zone.North Vietnamese troops also shelled a U.S. Naval patrol base at Cua Viet and the adjacent Marine Amphibious Tractor Battalion unit 8 miles southof the DMZ.THE PATROL BOATS scattered and directed gunfire on the Reds from the IJ.S. destroyer Morton. The Navy said the destrover's 5-inch shells touched off seven secondary explosions and suppressed the attack. No American casualties or damage were reported.Further north an enemy shell hit the forward stack of the U.S. destroyer Mansfield and killed one sailor and wounded two as she was shooting up coastal barges near Dong Hoi, 40 miles above the border.The Navy said the North Vietnamese shore battery landed 47 other shells in the water around the Mansfield, but return fire silenced it.Marines are making comparisons between the battle of Con Thien, a defen-A SENIOR noncommissioned officer who has been in and out of Con Thien many times complained it is not the kind of fight the Marines, jammed into muddy, sandbagged holes, prefer.“In the kind of war I’m used to,” he said, “when the Marines were bothered by a problem they'd go out and do something about it. They’d go out and dig that thorn out of their side.”Marines made two incursions in force earlier in the war into the southern half of the 6-mile-wide DMZ and rooted out some of the enemy, but they are staying put in this case. The southern half of the DMZ is part of South Vietnam.OTHER PLACES in the rugged frontier territory might be easier to supply and defend than Con Thien. But Marine officers feel the muddy red hills on which they are perched can be held. They consider Con Thien a valuable observation point.It apparently was believed when the decision was made to set up the Leatherneck base there that any guns across the way could be spotted and wiped out. But deep enemy emplacements in the jungled hills made them hard to pinpoint.Mortar and rocket crews can move fairly swiftly from one place to another, firing and hoping to get away before Americans locate their sites.