We are talking about the front lump I gather. Because the rear lump is always machined through, its concealment being the triggre plate, at least that is how it has been in the all the SXSs I have come across.

Solid pin boxlocks are by definition machined through, and the coverage of the front lump is via the bottom action plate. Some of the very best boxlocks are made this way, including some double rifles.