There does not seem to be any matching referance to OPEN DES limiting the Rate of Descent.
I reckon the "soft elevator" into the descent and subsequent ALT* wouldn't let the rate of descent rate get too high on a 1000' descent
The FCOM refers to MSN 0039 - 4551, so seem to applies to all models. Our really old ones have gone to a better place
, so will not have the opportunity fo some flight testing!