ZTG4503 a missing Blackmoor Book?

[Havard]

Dustin Clingman responded to this on FB:

I'll go back and check (it's been a bit), but the number wasn't reserved so much as intended for a subsequent reprint of the core campaign book. That would make sense in my mind. Were in not this, it would have been an error, there was no additionally contemplated product that remained unreleased that early on in the line.

-Havard