My guess (but that's all it is!) is that these are "tracking" numbers within a production batch, to keep an individual gun's parts together during manufacture. The fact that they are out of sight, and on the most closely hand-fit parts of the gun (loading tap), may support that. The "58" on the cover plate may indicate another model (the 45 underlever perhaps) had a similar plate.

A similar practice is seen on late-production HW 55's. On early examples (serial stamped on the rear receiver section early in the assembly process), all or part of the serial appears in four other places. But on late ones (serial stamped on barrel late in assembly) an arbitrary 3-digit number - unrelated to the serial - appears in those same spots.

An interesting question though! I need to study my lesser-model old Dianas more carefully.