These two sealings are labelled 1917-1920 in the ASI archives. They come from Punjab volume 26, earlier than the full 1921 excavation images which were in volumes 27 and 28. Why 1917? In the absence of more information it is hard to say much except that it was before 1921, and likely 1917-20 as the caption suggests.
But Sahni did not refer to it in his 1917 report, which one would have expected if it had been found prior to the commencement of excavations in January 1921.
Maybe they were supplied to the archaeological department before 1921, maybe even in 1917 but not catalogued? We are investigating and will post any updates if they become available.
"These photographs are of impressions of seals. They are not actual sealings of the Harappa Phase. The location of the original seals and the date of their discovery is not known."
- Jonathan Mark Kenoyer, 2021.