The number of Americans filing for unemployment benefits last week fell to the lowest level in more than 43 years, the government said Thursday, offering another sign that more workers enjoy job security. The Labor Department reported that 234,000 Americans sought jobless aid, a drop of 15,000 from the previous week and lowest since November 1973. The four-week average, a less volatile reading, dropped by 10,250 to 246,750, also the lowest since November 1973. The total number of people receiving unemployment benefits was 2.05 million, down 7.7 percent from a year earlier. Unemployment claims are a proxy for layoffs. They now have come in below 300,000 for 98 straight weeks. The low totals suggest that businesses are confident about their prospects that they were holding onto staff.