TL:DR – my current theory is about 1 in 3806 (that means, 1 Mageblood and 1 HH per 3806 Wretched sets, random unique belt beasts, Ancient Orbs, etc)
Note that it is widely speculated that Mageblood and Headhunter do not drop from monsters below monster level 75.
There is also speculation (which I don’t have enough data to say much about) that Tier 1 rarity items (Ryslatha’s Coil) might be considerably more common than this estimate indicates.
Spreadsheet just as an image
Speculated rarity (tier 0 to 2 belts combined into one entry):
Speculated rarity (only for the ~1 in 68 unique belts that are a rare belt, tier 0-2):
oh and the video because i forgot