Wasn't BHD the old British Horse Databsa you had to register with years ago to reg BD/BSJA/BE - was disbanded not long after I paid to join it! - if it is and your horse was registered with it it would be overstamped on black ink on your horses passport
I have 2 passports that where issued by the BHD so mine where never overstamped there lovely blue passports. I then had to have them overstamped & pages had to be added when the new rules for human consumption came in. I got Weatherby's to overstamp them as the BHD was no more.
If you have a BHD passport then yes the Number is the Passport number.