The Tektronix Type L is plug-in for 500-series scopes.
Type L is a Type K plus a 10x-gain AC-coupled preamp that extends the maximum sensitivity to 5mV/div. Bandwidth in a 545 is 30MHz at 50mV/div and above, 3Hz-24MHz at 20mV/div and below.
Type 53/54L was introduced in 1957 along with the 536 and its companion 53/54T sweep plugin. It was renamed Type L in 1959. It was the fastest 5mV plugin until the 1A1 in 1964. It was dropped after 1971 or 1972.
Late versions have a BNC connector instead of UHF.