This paper studies a possible structural origin of the finite vacuum propagation bound c∗ used in the Lorentzian bridge paper FBT08A. The aim is deliberately limited. We do not derive the numerical value of the physical speed of light, nor do we claim a complete microscopic formula for c∗. Instead, we isolate a dual-phase mechanism inside the Fracture–Berry–Tension (FBT) framework that makes the finiteness of the effective vacuum propagation bound geometrically natural. The argument combines two ingredients from the reduced dual-phase sector. The first is a positive minimal phase action quantum hphase > 0, arising from the quantised action lattice of the dual-phase torus. The second is a finite admissible structural multiplicity Nstr = 24, whose logarithmic form is ln 24. Taken separately, these quantities belong to different layers of the theory: hphase controls scale minimality, while 24 controls structural multiplicity. Taken together, and under an admissible readable-sector normalisation, they define a finite readable phase capacity Cphase < ∞ for the observable dual-phase sector. The main theorem states that, if the observable Berry readout in the vacuum regime is controlled by this finite phase capacity and the vacuum clock sector is nondegenerate, then the induced propagation-rate ratio is bounded above by a finite structural propagation constant κprop < ∞. At the level of the present paper, this finite constant is interpreted as the structural precursor of the effective bound c∗ appearing in FBT08A. Accordingly, the role of the present paper is not to replace FBT08A, but to support one of its central assumptions. FBT08A studies the geometric consequences of a finite nullpropagation bound. FBT08B explains why the existence of such a finite bound is natural from the point of view of the dual-phase readable micro-geometry.