L-10A

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 Tektronix L-10A Preliminary Manual Tektronix L-Series Narrow Band Spectrum Analyzers (All manuals in PDF format unless noted otherwise)

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