Yes,

53-55 have a door in them on the front of the heater, prior years did not. I have never measured them to see if there was a difference? Where exactly is it 3/4" off and what fit is affected?
Early version:
http://www.flickr.com/photos/advance-design_parts_co/8129448404/in/set-72157631868452754
53-55 version:
http://www.flickr.com/photos/advance-design_parts_co/8129313579/in/set-72157631868452754