Hi Ted,
If you look at the builder's photos of the various passenger cars while at the Jackson & Sharp plant in Wilmington, Delaware, you can see the Miller hook couplers. As for a date for conversion to Janney couplers, possibly comparing photos in the Kohler-Chesney books, as well as the Peter Barney books, might give you an approximate timeline after the Safety Appliance Act was enacted.
Dave Crow