files.txt

vbatts@bananaboat ~ (master *) $ sha256sum x.combined.tar.gz x-1.combined.tar.gz 
f283ba0d5fbc812dc27d26946e08d1ed9a49325c244b5dbfa51a37018030b327  x.combined.tar.gz
768ed169ddeccc1c8ccef6a8696c02a4cda1a135b05da47b205f074dcd9c4415  x-1.combined.tar.gz
vbatts@bananaboat ~ (master *) $ tar tvf x.combined.tar.gz 
drwxr-xr-x vbatts/users      0 2016-05-10 06:33 ./
-rw-r--r-- vbatts/users   1024 2016-02-18 18:10 ./5f70bf18a086007016e948b04aed3b82103a36bea41755b6cddfaf10ace3c6ef.tar
-rw-r--r-- vbatts/users 88473600 2016-02-18 18:10 ./6ff50884daa3ba41e6780e06bf2af92650840c52029c5e80f22f4d1c1f1a38a4.tar
-rw-r--r-- vbatts/users      277 2016-05-10 06:31 ./manifest.json
-rw-r--r-- vbatts/users     1620 2016-02-18 18:10 ./eca00f1249b3daa555d2c6e88dfb6a15c06f2f80e92ecb20f6bb7d8843deec15.json
vbatts@bananaboat ~ (master *) $ tar tvf x-1.combined.tar.gz 
drwxr-xr-x vbatts/users      0 2016-05-19 13:56 ./
-rw-r--r-- vbatts/users    557 1969-12-31 19:00 ./manifest.json
-rw-r--r-- vbatts/users   1667 2016-03-04 13:40 ./ddd5c9c1d0f2a08c5d53958a2590495d4f8a6166e2c1331380178af425ac9f3c.json
drwxr-xr-x vbatts/users      0 2016-05-19 13:56 ./37bd823246764341096127d5bf3533e66704a89a9dd3dca2914f3af6892c5d68/
-rw-r--r-- vbatts/users 237479424 2016-01-04 16:27 ./37bd823246764341096127d5bf3533e66704a89a9dd3dca2914f3af6892c5d68/layer.tar
-rw-r--r-- vbatts/users      1685 2016-01-04 16:27 ./e3cae2cffbcd01d16a3546ae5956db7fb03f78aca00ae66f9738ed14941039bd.json
drwxr-xr-x vbatts/users         0 2016-05-19 13:56 ./9a233237d70560774705931fc55fe1a3a4619cccf2d0a76671256080c2af6fdb/
-rw-r--r-- vbatts/users 212476928 2016-03-04 13:40 ./9a233237d70560774705931fc55fe1a3a4619cccf2d0a76671256080c2af6fdb/layer.tar
drwxr-xr-x vbatts/users         0 2016-05-19 13:56 ./768d4f50f65f00831244703e57f64134771289e3de919a576441c9140e037ea2/
-rw-r--r-- vbatts/users      1024 2016-03-04 13:40 ./768d4f50f65f00831244703e57f64134771289e3de919a576441c9140e037ea2/layer.tar

x.manifest.json

{
  "schemaVersion": 2,
  "mediaType": "application/vnd.oci.image.manifest.v1+json",
  "config": {},
  "layers": [
    {
      "mediaType": "application/vnd.oci.image.serialization.combined.v1+json",
      "size": 34121899,
      "digest": "sha256:f283ba0d5fbc812dc27d26946e08d1ed9a49325c244b5dbfa51a37018030b327"
    },
    {
      "mediaType": "application/vnd.oci.image.serialization.combined.v1+json",
      "size": 153571451,
      "digest": "sha256:768ed169ddeccc1c8ccef6a8696c02a4cda1a135b05da47b205f074dcd9c4415"
    }
  ],
  "annotations": {
    "AUTHORS": "Vincent Batts <vbatts@hashbangbash.com>",
    "SOURCE": "docker://docker.io/vbatts/slackware docker://docker.io/fedora"
  }
}

• are application/vnd.oci.image.serialization.combined.v1+json implicitly not compressed?
• is the digest of the uncompressed object?
• Is this valid? Despite that these two objects are unrelated?
• is config even needed in the manifest it the combined object includes one?

I still don't understand what you're trying to do. Can you explain to me like I'm 5?