It is usually controversial to include a partial layer in the layer count, because there is always a reasonable possibility this may be some contamination or other accidental feature. If you discern that the perforation is out-of-round, and that the unrounded part would have included the rest of the base layer, you could make a theoretical argument that it was formerly complete, but suffers from perforation abrasion and missing structure.

We do the same thing, basically, when we say that missing external layers were originally there, but have been ground-away—based on the structure of the glass and knowledge of conventional editions (that the bead otherwise resembles).

The uncontroversial interpretation is that this is a five-layer bead with a white partial base layer that may have been unintentional.

Drawn beads—particularly larger ones—were not "tumbled" until the 20th C. Prior to that time, the en masse technique was called the "a ferrazza" (or "stirred in a pan") method. This is covered in my article on Venetian beadmaking that appeared in BEADS three issues ago.

Jamey