After more research I found that 33322-60011 is the only attenuator that attenuates up to 130dB using 10, 20, 30, and 60 dB sections. So using any other attenuator would result in discontinuities when sweeping the signal amplitude since the attenuation that the 8648C tries to select wouldn't match what the mismatched attenuator gives.
With that in mind I successfully repaired my 33322-60011 thanks to a series of small miracles. For those Googling, the problems with the attenuator were:
Contact fingers not mating well with the bypass pads / attenuator pads Solution: clean pad mating surfaces with a q-tip and rubbing alcohol until a good connection is observed with an ohmmeter. [1]
Plunger o-rings missing / deteriorated Solution: ideally buy new o-rings, but being impatient I used a small amount of hot glue to secure the plungers with bad o-rings to their respective metal plates. Totally amateur but it worked. [2]
Solenoid wire was severed Solution: re-solder the very fine green wire back to its respective post
[1] Take a look at page 9 of this PDF showing attenuator internals. Note the gold flaps between each bypass surface, and how the ends of the gold flaps make contact with the bypass pad:
http://ve2azx.net/technical/HP_StepAttenuatorRepair.pdf[2] This video goes into detail regarding the o-rings:
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