These zoomorphic vessels appear to belong in a limited chronological range. Heqat, the frog goddess, was the propitious goddess of fertility and birth. The frog, through its metamorphosis, encouraged the Egyptians' hope of transformation in the afterlife.
Cf. R. Fazzini, Images for Eternity: Egyptian Art from Berkeley and Brooklyn, New York, 1975, pp. 13 and 134, no. 9 (from Naga ed-Deir, Grave N7304) for similar. For the choice of mottled stone imitating the skin of a frog cf. Exhibition Catalogue, Ägypten, Basel, 1997, pp. 41-42, no. 20.