I think the 5Hz to 50kHz spectrum analyser that David was referring to earlier might be the HP3580A. This audio spectrum analyser dates back to 1973.
The notes in the link below suggest it has a 1Hz RBW.
https://www.hpmemoryproject.org/wb_p..._b_page_04.htm
If you scroll down the link a bit further, you can see some stuff about the HP8568A. Back in 1978, this 1500MHz spectrum analyser really represented a huge step forward in terms of RF performance and features and it could be fully controlled via GPIB in an ATE system. I think a lot of the 8568 analysers were purchased for use in ATE racks. The high purchase price was presumably worth it, because it was able to speed up production testing and improve the quality/repeatability of the testing.