Besides `ar` as a sibiling observed, you might also be thinking of pixz - https://github.com/vasi/pixz https://github.com/vasi/pixz , but really any archive for

Besides `ar` as a sibiling observed, you might also be thinking of pixz - https://github.com/vasi/pixz https://github.com/vasi/pixz , but really any archive format (cpio, etc.) can, in principle, just put a stake in the ground to have its last file be any kind of binary / whatever index file directory like Zip. Or it could hog a special name like .__META_INF__ instead.