The conversion insert to allow you to use the Gen 1,2,3 recoil spring assembly with the Gen 4 Glock pistol. Made from 304 stainless steel . Use with the appropriate recoil spring assembly for your Glock model. Allows you to use a stainless steel rod with different weight springs, or just use the Glock OEM for the pistol. This is the perfect part for anyone having trouble with their 4th Gen Glock Guide Rod.