The Tektronix L-10A looks to be a plug-in made during the assimilation of Pentrix products into the Tektronix product line.

Tektronix L-10A
1 MHz to 36 MHz Spectrum analyzer
Tek L10-A

Compatible with 500-series scopes

Produced from 1965 to (?)

Manuals

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.

Photos