The Svetambara Jain canon is not a book that was written. It is a body of teaching held to descend from Mahavira through a chain of teachers, transmitted orally, and repeatedly at risk. Jain tradition remembers several occasions on which the community had to gather what remained of it, and one of those occasions is Mathura, under the elder Skandila.
The account is of a community that had lost reciters and lost material, coming together to establish what could still be recovered and in what order. What Skandila's assembly produced is called the Mathuri Vacana, the Mathura recitation, and it is one of the recensions the later canon rests on.
Mathura is a plausible place for it. It was a major Jain centre and the archaeology proves it: the Kankali Tila mound has produced a large body of Jain sculpture and inscribed votive tablets from the early centuries of the common era, which is independent evidence of a substantial, well funded and long established Jain community there of exactly the kind that could convene something like this.
What the tradition does not preserve is a clear account of the assembly's procedure, its participants or its date, and the surviving canon does not carry marks showing which readings came from which recension. So the event is remembered, the place is corroborated, and the details are gone. The atlas records the date as an open question rather than repeating a traditional figure as though it were established.