Multiple Sprinco retailers have this as part of the ad copy for the green spring: "Over gassed heavier caliber guns with rifle length tubes may be addressed with our Red or Orange carbine springs and standard length 3.25” carbine buffers along with a spacer to yield the appropriate tube depth for the longer BCG’s. Our line of enhanced spacers will be available in the 3rd Quarter of 2019."

There is also a mention on https://thearmsguide.com/8736/buildi...ity-explained/

Springco has designed a new series of ultra-high density co-polymer spacers which will soon be available on their website. They will permit the use the many weighted carbine buffers and Sprinco “Red’ and “Orange” springs in your .308 rifle length extension tubes to provide many different combinations to match your particular applications.
I can't find anything else about these. Were they cancelled?