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 plugin scope line after 1974. Not particularly well calibrated in either frequency, bandwidth, or amplitude, it was no match for HP's 141T/8556A/8552B combination as performance was concerned, but it was a small fraction of the price.
Although Type 1L5 can operate in any classic plugin scope*, it is by far the most useful in the Type 549 storage mainframe, where it's possible to sweep slowly and take advantage of the narrower resolution bandwidths.
* 531/535/541/545 need a wiring mod to alleviate a vertical amp instability.