All printed editions (1940, 1971 and 1981) of the SM do not mention any manuscript source, either in the Premiss (altogether absent in the first two) or in the footnotes. The 1981 edition, though endowed with a nice Introduction, does not mention any source at all, be it manuscript or printed. Hence, I speculated that they all copied from an earlier edition and started looking for it. On-line catalogues and resources bear no evidence of any earlier edition, but I am sure this is no conclusive evidence, since this might have been published in a small typography, perhaps only for religious purposes.
Today, I finally found the following lines in Paṭṭābhirāma Śāstri's (Sanskrit) Preface to the 1981 edition (my translation):
Eighty years ago, in South India, in Kāñcipurī, a book called Seśvaramīmāṃsā has been published through the Printing House Sudarśana, once edited by the honourable Prativādibhayaṅkāra Anantācārya (p. iii).
As expected, I could not find any information about this edition. The Sudarśana Printing Press seems to have been active in Kañcipuram at the beginning of the 20th c. (several books published by it are listed by the Digital Library of India, all around 1900-1905). It might have published mostly Vaiṣṇava works, but I have not found any conclusive evidence about it. Do readers know better?
Do readers know where is it likely to find its books? In Chennai? In Kancipuram itself?