Two days later, General Pollock amplified this directive by underscoring the necessity for holding these two positions, plus eight more he considered vital for sound tactical defense. These additional positions, from west to east, were Hills 86 and 37 in the KMC sector; Hills 56 and 48A in the center sector; and the outposts then known as Allen, Clarence, Felix, and Jill, all the responsibility of the right regiment.[223] [223] When the 7th Marines took over this sector from the 5th in early September, the names changed to Carson, Vegas, Detroit, and Seattle respectively. COP Bruce was also redesignated as Reno. Since the old names of the outposts were well known to the enemy, for purposes of security it was decided to identify them differently. U.S. cities were selected. Although the eastern part of the division main line thus contained at this time more key hills than any other Marine sector, much of the increase in Marine patrol and ambush activity took place in No-Man’s-Land forward of the middle frontline regiment. Of the two JAMESTOWN sectors manned by U.S. Marines, the one in the center of the division area offered better ground for infantry operations. On the divisional western flank, the Korean Marines conducted frequent infantry-tank patrols during the second and third weeks of September, but the enemy opposite the KMCs initiated little ground activity. Instead, the Chinese relied upon their supporting weapons to provide the contact. For a seven-day period ending 19 September, a total of 2,375 enemy rounds had fallen in that regimental sector, an average of 339 per day. Nearly a third had been in the vicinity of Hill 36. Before sunrise on the 19th, a Chinese infantry company had crossed the Sachon in the vicinity of the railroad bridge. Once on the east side, the enemy soldiers concealed themselves in caves and holes, remaining there until dusk. Then, when they came out of hiding, the Communists held a briefing and organized themselves into three attack groups. As these advance infantry elements approached their objective, OP 36, other reinforcing units were prepared to seize OP 37, to the east, and OPs 33 and 31, to the south. Artillery and mortar preparation supported these diversionary attacks.