Unlikely sign at a 54-Across

• Like many showings of a hit musical, briefly
• Sellout warning (Abbr.)
• Capacity-exceeding letters
• Like hit shows
• Big hit indicator
• Sold out sign
• Sign for the seatless
• Sign of a sellout, for short
• B'way producer's favorite sign

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
1941, initialism (acronym) for standing room only.