You will find much of the information outdated, missing, and even a few tidbits incorrect at the AH Trainer's website.
Aces High Stats (damage)
Japan 20mm Type 99 Mk I HE 3.42
Japan 20mm Type 99 Mk II HE 3.85
Japan 20mm Ho-5 HE 3.35
I tested and verified all three myself. Of course, RoF and trajectory minimizes any damage differences.
FWIW, I have tested each and every .50cal and higher weapon in AH since the first of the year, including the new British rocket damage, 28cm rocket (from the SdKfz 251), new 4.5in rocket damage, etc, etc. I have it all. I've offered it to the AH Trainers to use and post the data as they wish on the website but it has fallen on deaf ears. I'm under the impression that others have tried to help update the information but to no avail.
Anyone else know how much difference the damage is between the Hispano Mk II and Mk V versions?