Sometimes it referred to as OOR (Out Of Range)
You can still work out the S/N from the BB and release quantity.
You tend to find OP release/d models for promotional purposes, (gifts, competition, sponsorship etc) They also produced prototypes and pre release models which ended up with staff or customers etc and then move into the open market.
Just because the range might say 500 there could be 525, though usally this info is available from various sources