33120A is obsolete, and the front panel mechanical rotary encoder, part# 0960-0892 (uses switch contacts) is no longer available from Keysight.
I found a Bourns part with same footprint, The PEC16-4215F-N0024 available at Newark for ~$1ea. This has detents, original did not, but who cares? The instrument won't. Not sure, but this is probably usable in several old HP/Agilent instruments, like the 34970A, which we have several of.
To anyone actually acting on this info, note corrected part# above. I originally posted the wrong part # ending in -S0024 which has a momentary push switch built in, and stands about 1/8" higher off the circuit board- won't fit in space under front bezel. Lucky they're $1, I'm stuck with 10 wrong pieces.