"...Plastic grips..." Plastic or Bakelite? Bakelite is an early form of plastic that dates from 1909, but not exactly plastic as we know it. It wasn't petroleum based.
"...I have never seen it..." Ever ask grandma to show it to you?
In any case, Colt will send you whatever info they have. Not exactly cheap though. $100 for grandma's Colt.
http://www.coltsmfg.com/cmci/historical.asp
From this page, there's an 1877 and '78 DA revolver.
"...Colt DA..." This is the model. You're likely right about it being a .41 Colt.