H series used to only go up to 24" in depth. Some manufacturers started to manufacture H-series to 30" and labeled the 26, 28 and 30 "DH" until SJI included them as standard "H" series in their tables.
That's an oddball occurrence, and would be well if you have the manual to forward the material to SlideRule, or post it here, so he could post it on his website, should he so desire.
Could come in handy in the future for someone here.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask