Lucas McCain is wearing Wrangler jeans that weren't invented until 1947. The original company that became the Wrangler Corporation was founded in 1904 but Wrangler jeans themselves were not sold until 1947. The Rifleman was filmed for the 1880's time frame
Levi's would have been the brand name jeans of the day because Levi Strauss & Company started telling their denim jeans in 1853.