You might be thinking of ar, the classic Unix ARchive that is used for static libraries?

The format used by `ar` is a quite simple, somewhat like tar, with files glued together, a short header in between and no index.

Early Unix eventually introduced a program called `ranlib` that generates and appends and index for libraries (also containing extracted symbols) to speed up linking. The index is simply embedded as a file with a special name.

The GNU version of `ar` as well as some later Unix descendants support doing that directly instead.

Besides `ar` as a sibiling observed, you might also be thinking of 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.