128K Mac without signatures inside
It appears that the signatures were only on the initial run of the 128K Macs.

I dug this up from DigiBarn Weird Stuff


  These signatures were molded into the case and included here
  (gradually disappearing) for several generations of Macs through to
  the SE


And from the Obsolete Computer Museum..


  The first Macs had only 128k RAM and used single-sided, 400k floppy
  drives. The signatures of the original Mac designers are molded into
  the inside back of the case.


It's possible that this may be a latter generation 128K Mac without the signatures.

You also say that you're "pretty sure" the rear shell hasn't been replaced.  However, you didn't say that you were certain.  It's entirely possible it was replaced for service at some point.