Erebor went through several phases of occupation by the Dwarves: it was first inhabited between III 1999 and III 2210, and then from III 2590 to III 2770, and was finally re-established in III 2941. We aren't told which of these periods saw the introduction of the Ereborian mode of the Cirth, and our only definitive example comes from the Book of Mazarbul, written after all three of these periods and therefore theoretically compatible with any of them. We assume here that the Ereborian variant derived from the earliest period of Erebor, though it is entirely plausible that it arose during the second period, during which Thrór was King under the Mountain. The final phase, after Erebor's recovery from Smaug, seems too recent to be a realistic possibility, but even that fits with the minimal evidence we possess.