@ Willie
I'm not so sure a CS500 isn't in Bombardier's future plans. They've copyrighted it and the C Series type rating is the BD500 (for all variants).
I wouldn't read too much into the designation on the type certificate. The first of the "BD" designations was the Global Express, which took BD-700 (likely because it was a bigger aircraft than the CL-600, which it was supplanting as the company's big business aircraft). The Challenger 300 (ne Continental) is the BD-100, as indeed is the Challenger 350. So there's no real link to the marketing designation from the Type certificate ID.
At the time of Cseries launch, when the type certificate number would have been applied for, there was no BA aircraft numbered "500", but there were already aircraft using most of the other "hundreds" either in the TCDS or the marketing name. Likely it was simply the "untaken" number, nothing more.