The K120 is a fine speaker reminiscent of the surf era when lead guitar was bright, clean, and loud
Has to be one of the best guitar speakers ever built so I hope it suits your purpose
The frequency response curve approximates the equalization needed to produce an even SPL across the range of string frequencies. It is in effect a built-in natural equalizer that offsets the signal amplitude ratio from bottom open E to top E on the 12th fret. Guitar amps try to achieve the same effect by using small-value capacitors in between stages.
The specs are attached
There is a detailed discussion here with response curves https://www.audioheritage.org/vbulletin/showthread.php?31943-K120-re-coned-with-McKenzie-kits
To validate the curves for your unit you would need to determine any differences in cone construction between the JBL and McKenzie kit version
Is a great speaker
High SPL, high power rating, wide frequency response, Alnico magnet and cast aluminium frame, treated linen surround – what more could you ask for?