Well, I just became a believer. After reading the lead-in post, I went to the following WEB site:
http://www.boschautoparts.com/Global/Contact
then called the phone number there -> 888-715-3616. Opened an issue and mins. later, a Bosch employee called me.
According to him H9DC & H9DCO have been discontinued.
They have BOTH been replaced by --> HR9DC stock # 7578.
If you want to continue using the discontinued plugs, you'll need to run to your local retailer and buy up whatever stock they have. Once they're gone, they're gone.