Okay, I have today had a little more time to investigate what actually happened:
1) My 8593A has option E02, "Digital Radio Test System" in ROM.
2) Per attached image, there is a "HP 08593-10001 ROM CARD", which contains various sub-programs that can be assigned to the right-hand soft menus. This screen is reached by clicking [MENU] button + "Mode Loader" soft button.
3) These sub-programs can be deleted from memory by "purging" them. I accidentally did so for program '7' "Frequency Counter".
4) I was mistaken, there is no frequency counter board in the slots.
So, the question now is: Where can I get my hands on the "HP 08593-10001 ROM CARD"? These options seem quite cool.
The E02 option and this menu is described here (8593E so may have different options. The 8593A PDF is scanned images so not searchable - i'm still looking through those.
http://www.av.it.pt/medidas/Data/Manuais%20&%20Tutoriais/15%20-%20HP%20Spectrum%20Analyser%208593E/HP8593E%20Manual%20Supplement.pdfThanks,