Ms. Beimert's uploads are from the Peters anthology: Franz-Album Ausgewählte Lieder Band I, assembled around 1890. These are the low-voice versions. Songs were typically published in low/high, and middle editions. Not all songs were transposed, so sometimes the songs are identical.
I have volume two of the Peters reprint with the rest of the songs. I'll scan them in the next few weeks.
I think the first number 5292 (high voice) and 5621 (low voice) is the Peter's catalog number and the second number is the Siegel number. The publisher would only engrave one title page which could be put around the volume, so the Peters title page I have has both Peters catalog numbers (but no Siegel).
According to New Grove, the set was published in two sets: No's 1-6, and No's 7-12. My understanding is these were always sold in paperback, and the purchaser would have the songs bound together in an album, so you only bought what you were interested in. Each set would be a signature. That's why they come in groups of six songs, and each takes exactly 16 pages (ie, four sheets sewn together) and/or a cover page.
Just figured it out: the second number is the song sequentially: so 1644-1655 are the 12 songs of the set.
Mystery solved! I consulted the 1903 Siegel catalog. The songs were published in multiple versions: Complete songs for high voice (in two Hefts, ie. signatures for 3 marks each) Complete songs for low voice (in two Hefts for 3 marks each) 1644-1656 Songs sold separately (1 mark each) 5684-5695 Songs sold separately for low voice (1 mark each) [presumably one song would not need to be transposed]
Then an anthologies with translations by Elizabeth Rucker spread over two volumes: 5292: Band 1 (volume 1): original keys (3 marks): includes No 1-5, 7, 10, 12. XXXX: Band 1 for middle voice 5621: Band 1 for low voice
5021: Band 2 original keys (3 marks): includes No. 5, 8, 9, 11. 5764: Band 2 low voice [No middle voice for volume 2 in the catalog]
In order to make the middle voice version (which is probably what Ms. Beimert uploaded), songs 1 and 7 used the high voice engraving and 2-7 the low voice. No's 1 and 7 are both one page songs and are on pages 3 and 10 of the anthology, meaning the same side of the second sheet of the first signature. That's why the anthology skips song 6 in the first volume: the back side of the signature had song 7 on it because the plates had been re-configured.