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It's exactly 3/4", the difference between a5 and carbine internal tube length. Allows use of carbine buffer and spring in A5 tube. I use the a5 tube just for the extra lop, gives perfect max 13 3/8" with a sba3 pistol brace, or 14.5"with Magpul CTR.
The spacer that comes with the scs is for a rifle length tube. This 3/4" spacer is for an a5 tube, or an armalite spec ar10 tube, both have the same length and are 3/4" longer than a carbine tube. 308s can use any buffer tube length and the scs, or a carbine buffer/spring, with the right spacer.
JP Enterprises does not provide a length spec. I would recommend contacting them directly.
https://www.jprifles.com/2.1.php