The purpose of this paper is twofold. The first is concerned with the hidden properties of ur-baryon dynamics deduced from the previous analyses I and II of meson mass levels and from work by several authors. The previous results are discussed from the viewpoint of neutral vector theory with a pair suppression hypothesis. These will be re-expressed in terms of the Up(3) ⊗Up(3) symmetry, although it adds no essentially new result. These considerations are extended to the mutual interactions of meson nonets with the (tt) structure, and a dynamical interpretation of the A selection rule is presented. It will be conjectured that the dominant part of the effective trilinear vertices is of the Tr(φaφbφc) type and not of the Tr(φa)Tr(φbφc) or Tr(φa)Tr(φb)Tr(φc) type, where φi(i = a, b, c) denotes a relevant meson nonet. The second purpose is to study the decay properties of the L = 1 meson systems, taking Tr(φaφbφc) type vertices and a common 1-8 mixing angle for the L = 1 mesons determined from the linear mass difference equations discussed in II. The calculated results as well as comparisons of them with available data will be discussed in detail.