This link should bring you right to that play. [ok.ru]

And here's what I see:

The Rams break huddle with about 16 seconds left on the play clock. They get lined up, Stafford in shotgun with Williams to his right.

It does not look like Stafford changes the play, but with 5 seconds left Stafford moves Williams up, right behind the line between Shelton and Dotson, presumably to make it easier for Williams to pick up the blitz.

Kupp goes in motion, right to left, with 3 seconds left.

Stafford waits for Kupp to clear, then claps his hands for the snap with 1 second on the play clock.

Dotson does turn & see Stafford clapping, and he does signal Shelton to snap the ball, and Shelton does snap it. But it's way too late by that time.

And I agree 100% with your conclusion. Stafford should have called time out - it should have been clear even with 5 seconds left on the play clock that they wouldn't be able to get that play off.

I wonder if the Rams had some mismatch lined up that made running that play imperative?

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AlbaNY_Ram