These stocks are quite rare to find, these were covered under US Patent by Robert Roy who worked for Colt in 1964 and these CAR-15/XM607stocks saw limited use being replaced by the XM177E2. Around 2013 a small production run of some repro XM607 stocks were produced in Texas, these were a close copy but still not quite the same. The lever inside the buttstock locks or unlocks stock.
photos show original (left) and repro and buttstock leverAttachment 111351Attachment 111352Attachment 111353