The Tektronix 1L5 is a low-frequency spectrum analyzer for 500-series scopes.
Its center frequency is set on a trio of rotary switches and can range from 50Hz to 990kHz. Minimum resolution bandwidth is 10Hz. It has a 1 meg input resistance.
Type 1L5 was sold from 1967 or 1968 until the end of the classic plug-in scope line after 1974. Not particularly accurate in either frequency, bandwidth, or amplitude, it was no match for HP's 141T/8556A/8552B combination in performance, but it was a small fraction of the price.
Although Type 1L5 can operate in any classic plug-in scope*, it is most useful by far in the Type 549 storage mainframe, where it's possible to sweep slowly with the narrower resolution bandwidths.