If the bbl finish is original,,I'd guess the 55407H number is the first to have been applied of the two number stampings on the bbls.
That number has soft solder in the stamp impressions and the impressions has it's raised imprint filed or polished flat with the bbl surface.
That tells me the stamp was on the tube when it was assembled along with the ribs & soldered together.
Then the assembly cleaned up of excess solder before the finish applied.
The other number (55470) looks to have been stamped into the surface before the finish applied (in the white), and after any soldering work was done (no run over solder in the impressions) and also no polishing down of the raised portion of the imprint(s) was done after it was stamped.
The different sets of stamps used on the bbl numbers has already been noted.
55470 on the frame and forend was done with yet a different set of stamps than the 55470 on the bbl.
Just observations of the stampings themselves,,I don't have any idea of how/when Lefever (or Ithaca) marked the bbls and parts during mfg & assembly.
