L-10A
The Tektronix L-10A looks to be a plug-in made during the assimilation of Pentrix products into the Tektronix product line.
It was introduced in 1965 according to TekWeek Jan 7, 1966. The time line seems to be Pentrix L10, then Tektronix L-10A, then Tektronix 1L10.