My wife's passport renewed in Santiago begins with "RD".The number changes with the place of issue as well. So passports issued in Santo Domingo start with SD then numbers, Puerto Plata is PP I believe, and so on with other passport offices throughout the country.
Those issued from abroad have different letters at the beginning as well.
My wife's passport was issued 1st quarter of 2020 by Santiago Authority.My husbands old passport, issued in San Pedro begins with SP, whereas the one issued in Toronto begins with TO.
His brothers begins with SD. Maybe it depends on how long ago it was issued as well?