Tower Tech notes say the 251 is compatible with the 601/611 series gyros only.

Futabarc.com has it listed as being compatible with the 611 and 401

Since the 611 operates with a 760us pulse width, and the 401 with the standard 1520us I can't see how it could be compatible with both.

It was a typo. It should have read: "GY601 and GY611".

The page has been corrected.

Thanks Bill! Gonna be some cranky campers, I've seen people buying them thinking they work with the 401.