Carl's bomb-handling skill, stalled out until the Agent Provocateur specialty granted a +2 boost.
Explosives Handling is listed among the benefits of the Agent Provocateur specialty, which grants +2 to it plus faster explosives training and +1 skill level to a Sapper's table.
Carl notes that his explosives handling is another skill that had stalled out, and that the boost would send him over the hump — one of the reasons he chose the specialty over Revolutionary and Guerilla.
The core crafting skill behind Carl's bomb-making.
Also appears in books you've already read — spoiler-safe to their endings: