The Black Hack 1e has an optional rule called "Original Armour". It reads like this:
The traditional concept of ascending AC bonuses can still be used with The Black Hack. Attacks are still made by rolling below attributes (such as STR and DEX) however the AC bonus of the target is added to the roll.
For example in ascending AC systems Leather typically adds +2 to a base of 10 (giving you AC12) – with TBH you would roll an attribute test to see if you hit or are able to avoid taking damage, and add +2 to the dice roll. The quick way to read AC from existing resources is just use the last number as a bonus.
But to avoid taking damage in this edition of the The Black Hack one also has to roll under a stat. In which case it makes no sense, in my opinion, to add +2 to the dice roll. Rather, it makes sense to add +2 to the target number. Is my interpretation correct?
For reference, the Black Hack 1e SRD is here. And the optional rules, including the one in question, are here.