Another example. metaflac on the 'source' .flac file:
    comment[0]: CDDB=e60ffa10
    comment[1]: GENRE=Rock
    comment[2]: ORIGINAL_DISCNO=1
    comment[20]: album=Camper Vantiquities
    comment[18]: albumartist=Camper Van Beethoven
    comment[13]: albumartistsort=Camper Van Beethoven
    comment[25]: artist=Camper Van Beethoven
    comment[30]: artistsort=Camper Van Beethoven
    comment[12]: asin=B0001BRQ1Y
    comment[28]: barcode=077771321122
    comment[19]: catalognumber=0777 7 13211 22
    comment[9]: date=1993-03-23
    comment[10]: discnumber=1
    comment[27]: disctotal=1
    comment[6]: label=I.R.S. Records
    comment[22]: media=CD
    comment[8]: musicbrainz_albumartistid=0ee7e64d-90ca-406b-b59a-3e551c556dbe
    comment[16]: musicbrainz_albumid=76c70a24-7fb9-4714-a34c-8b1a7d0d98d2
    comment[21]: musicbrainz_artistid=0ee7e64d-90ca-406b-b59a-3e551c556dbe
    comment[26]: musicbrainz_releasegroupid=9f5aedb7-f821-3682-aec1-1797594b5493
    comment[29]: musicbrainz_trackid=21115ab2-9b37-4f16-93c3-7f32f3300bbb
    comment[14]: originaldate=1993-03-23
    comment[4]: releasecountry=US
    comment[17]: releasestatus=official
    comment[23]: releasetype=album
    comment[24]: releasetype=compilation
    comment[15]: script=Latn
    comment[3]: title=Ice Cream Everyday
    comment[5]: totaldiscs=1
    comment[7]: totaltracks=16
    comment[31]: tracknumber=14
    comment[11]: tracktotal=16

eyeD3 on resulting mp3:
Comment: [Description: CDDB] [Lang: XXX]
e60ffa10
Comment: [Description: ORIGINAL_DISCNO] [Lang: XXX]
1
Comment: [Description: releasecountry] [Lang: XXX]
US
Comment: [Description: totaldiscs] [Lang: XXX]
1
Comment: [Description: label] [Lang: XXX]
I.R.S. Records
Comment: [Description: totaltracks] [Lang: XXX]
16
Comment: [Description: asin] [Lang: XXX]
B0001BRQ1Y
Comment: [Description: originaldate] [Lang: XXX]
1993-03-23
Comment: [Description: script] [Lang: XXX]
Latn
Comment: [Description: releasestatus] [Lang: XXX]
official
Comment: [Description: catalognumber] [Lang: XXX]
0777 7 13211 22
Comment: [Description: media] [Lang: XXX]
CD
Comment: [Description: releasetype] [Lang: XXX]
album
Comment: [Description: releasetype] [Lang: XXX]
compilation
Comment: [Description: musicbrainz_releasegroupid] [Lang: XXX]
9f5aedb7-f821-3682-aec1-1797594b5493
Comment: [Description: barcode] [Lang: XXX]
077771321122

UserTextFrame: [Description: MusicBrainz Album Artist Id]
0ee7e64d-90ca-406b-b59a-3e551c556dbe
UserTextFrame: [Description: musicbrainz_albumartistid]
0ee7e64d-90ca-406b-b59a-3e551c556dbe
UserTextFrame: [Description: MusicBrainz Album Id]
76c70a24-7fb9-4714-a34c-8b1a7d0d98d2
UserTextFrame: [Description: musicbrainz_albumid]
76c70a24-7fb9-4714-a34c-8b1a7d0d98d2
UserTextFrame: [Description: MusicBrainz Artist Id]
0ee7e64d-90ca-406b-b59a-3e551c556dbe
UserTextFrame: [Description: musicbrainz_artistid]
0ee7e64d-90ca-406b-b59a-3e551c556dbe

OTHER Image: [Size: 52596 bytes] [Type: image/jpeg]
Description: 


Note the duplicated 'MusicBrainz Artist Id' vs. 'musicbrainz_artistid' and 'MusicBrainz Album Id' vs. 'musicbrainz_albumid'.

Picard apparently merges those into one tag, with ';' separating the two values

ubuntu 13.10 and sound-converter  2.0.4-0ubuntu1