The datum shift reference in the ESRI doco (http://edndoc.esri.com/arcobjects/9.1/default.asp?url=/arcobjects/9.1/ComponentHelp/esriGeometry/esriSRGeoTransformation3Type.htm) points to an EPSG code for the shift (1573). If you look this up in the EPSG database (http://www.epsg.org/) , it references a NTV2 Canadian Transformation (specifically for Quebec as mentioned above.) The funny thing is, this grid should not work outside of Quebec. In searching through all of the NAD27 -> NAD83 shifts (the mathematical ones anyways), nothing comes close to the shift (~ dx=60.0, dy= 108.2, dz=192.600000).