I always thought there was one and only one PDP-6 at MIT.  The one
running ITS at the AI Lab.  But then I found a post to alt.sys.pdp10 by
Marc Crispin:
  I did meet the MIT AI and DM PDP-6s 
Digging further, I found Project MAC progress reports mentioning this
Dynamic Modeling PDP-6.
Page 59 in 
http://www.dtic.mil/dtic/tr/fulltext/u2/732767.pdf
  at Project MAC, both the Multics GE 645 system and the
Dynamic
 Modeling and Computer Graphics Groups' PDP-6/10 system are network
 hosts. 
Page 60 in 
http://www.dtic.mil/dtic/tr/fulltext/u2/756689.pdf
  Work on the Dynamic Modeling System began,
effectively, in October,
 1969, when a used PDP-6 computer with 32K words of memory was
 delivered to Project MAC. 
Does anyone know more about this PDP-6?  Did it ever run ITS, like its
PDP-10 successor?