This information might be helpful to those with 51J/R388 receivers. It took me a while with the military manual and diagram to get the slugs puzzled out. My conclusion was that there are actually three different slugs used.
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SLUGS FOR 51J/R388 Receivers
L104, 105, 106, 107, 108, 109 111, 112 and 113 ARE THE SAME 3-13/16" L x .256"D
L101, 110 ARE THE SAME 4-9/16" L x .255" D
L102, 103, 114, 116, 118 ARE THE SAME 3-7/8" L x .25" D
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You might be interested in the diagram I made up some time ago:
If you're interested in it download it and/or print it out.
Ian, K6SDE