But how the old iPads held up a year after their release — and in the face of growing competition — will tell investors a lot about the lasting power of the new ones.

Last year’s iPads started off fiscal 2013 with a bang, selling nearly 23 million units in the holiday quarter. But sales fell off sharply in fiscal Q2 and Q3 — the first spring/summer without a new iPad — and we’ll find out how well they did in Q4 on Oct. 28, a week from Monday.

Meanwhile we’ve collected Q4 estimates from three dozen Apple analysts — 17 professionals and 19 amateurs — and summarized the results in the attached bar graph.

The overall picture they paint is of iPad unit sales that are flat. Flat year over year. And flat sequentially, from one quarter to the next.

Not that all the analysts agree. Far from it. There’s a gap big enough to land a spaceship between the top and bottom numbers, between a high of 18 million and a low of 11.5 million.

And this time, for reasons that escape me, the amateurs who usually guess high have come in low and the more cautious pros have come in high.

We’ll find out who was closest to the mark when Apple reports its earnings for the last quarter of fiscal 2013 after the markets close on Monday, Oct. 28

Below: The individual analysts estimates, with the pros in blue and the indies in green. Thanks as always to Posts at Eventide‘s Robert Paul Leitao for pulling together the Braeburn Group numbers.