Analysis of logs show that there are in fact some files with m1v1 too (not just m2v2),

which is documented to be MPEG-1 video.  Adding fourcc to isom.c.

Originally committed as revision 19180 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/isom.c

@@ -117,6 +117,7 @@ const AVCodecTag codec_movvideo_tags[] = {
     { CODEC_ID_H264, MKTAG('a', 'v', 'c', '1') }, /* AVC-1/H.264 */
 
     { CODEC_ID_MPEG1VIDEO, MKTAG('m', 'p', 'e', 'g') }, /* MPEG */
+    { CODEC_ID_MPEG1VIDEO, MKTAG('m', '1', 'v', '1') },
     { CODEC_ID_MPEG2VIDEO, MKTAG('h', 'd', 'v', '1') }, /* MPEG2 HDV 720p30 */
     { CODEC_ID_MPEG2VIDEO, MKTAG('h', 'd', 'v', '2') }, /* MPEG2 HDV 1080i60 */
     { CODEC_ID_MPEG2VIDEO, MKTAG('h', 'd', 'v', '3') }, /* MPEG2 HDV 1080i50 */