Apparently the RS3 came in 3 phases, I found this on the internet:
The Phase 1 RS3 came without any car-body filters. However, during 1950-1953, Alco improved the air handling inside the hood of the RS3 by using such filters. Phase 2 units are recognizable by the air filters running along the edge of the roofline on each side of the carbody. Further improvements led to two sets of stacked air filters on each side of the carbody. These RS3s are designated as Phase 3 units. Some other changes, such as angled numberboards at the carbody corners, are also evident on Phase 3 RS3s.
Here's some line drawings that may or may not coincide with what was written above:
http://www.trainweb.org/willstrainart/Alco_line.htm