Here is test case

$ touch test
$ ar -crfD xx
$ ./busybox_unstripped ar -x ./xx
ar: unrecognized file type


It works ok if the archive is produced using ar -crfU

It seems that the file modes change from 100644 to 0644 in archive header
which confuses busybox ar where its checking 

libarchive/data_extract_all.c

    /* Create the filesystem entry */
      switch (file_header->mode & S_IFMT) {
      case S_IFREG: {
...


and fails this check and reports the above mentioned error.