You could argue it’s implied that a-z sequence contains x. But I agree that it’s a bit ambiguous.

Here’s what a better notation would look like:

(1-x)(2-x)(3-x)…(n-x) for x from N

Or

(b0 - x)(b1 -x)(b2-x)…(bn-x)

This way, both Bn and X could be any number and not just natural ones.