ABILENE, Kan. (AP) ¿ Duckwall-ALCO Stores Inc. said Thursday that sales at stores open at least a year dropped 4.2 percent in August, hurt by a shift in the Labor Day holiday and first-of-the-month paychecks. Total monthly sales fell 4 percent to $33.1 million, the discount store operator said. For the year to date, sales at established stores edged up 0.4 percent, while total sales increased 3 percent to $270.5 million. Same-store sales, or sales at stores open at least a year, are a key indicator of retailer performance since they measure growth at existing stores rather than newly opened ones. Duckwall-ALCO has 257 stores. Shares fell 26 cents to $16.88 in morning trading.