Baptismal font in the Catholic church on Inis Meáin, Aran Islands, Galway.
From the Catholic Encyclopedia:
The font, if not of impermeable stone, must be lined with metal; it must be used exclusively for baptism, and to guard it against profanation, securely covered and locked. Frequency of thirteenth-century legislation on this point throughout Northern Europe reveals the prevalence of a passing superstitious belief in the magical efficacy of the font and its waters.
No, that’s not much of a lock — but enough to comply with the age-old belief that the baptismal waters needed to be protected.