The Tektronix Type L is plug-in for 500-series scopes.
Type 53/54L was introduced in 1957 along with the 536 and its companion 53/54T sweep plug-in. It was renamed the Type L in 1959. It was the fastest 5mV plug-in until the 1A1 in 1964. It was dropped after 1971 or 1972.