GregLamberson, I recently purchased both, and am not familiar with either one yet, AND don't have copies at hand, but B30.20 is more of a "use and ownership" spec, not useful for design, while BTH-1 is a design spec (standard?).
Both cover far more than spreader beams and padeyes, etc, such as vacuum devices, electrical systems, mechanisms, etc.
I discovered BTH-1 while in the process of ordering B30.20
Regards,
Mike