{Collins} 51S1 Audio Mods and Tube Shields

Does anyone on the list know of 51S1 audio mods that give a Serial #10XX receiver better sounding audio than one with a serial #6XX?   Also, and I'm hesitant to ask, is there a significant difference between the IERC black type tube shield and the standard EBY plated type?
Sponsored by the Collins Collectors Association http://www.collinsradio.org
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