= Solution
Base again generates $\mathrm{PhS^-}$ catalytically. In $\mathrm{Ph_2P(O){-}CH{=}C{=}CH_2}$, attack at the central <allene> <carbon> moves the adjacent C=C pi <electrons> onto the <phosphorus>-bearing <carbon>. This makes the stabilized alpha-phosphoryl carbanion
$$
\mathrm{Ph_2P(O){-}C^-H{-}C(SPh){=}CH_2}.
$$
<protonation> at that <carbon> gives \b[$\mathrm{Ph_2P(O){-}CH_2{-}C(SPh){=}CH_2}$], the second product drawn above. The remaining double bond is terminal, so no E/Z pair is possible. The new <sulfur> bond is at the central <carbon> of the original <allene>, not at the terminal methylene.
This regiochemistry places the developing negative charge beside the <electron>-withdrawing phosphine-oxide group, instead of at a less stabilized vinylic position. It is an ionic addition/<proton>-transfer cycle, rather than radical <thiol>–ene chemistry. <proton> donation by another <thiophenol> molecule regenerates <thiolate>. One <thiophenol> is incorporated without loss of atoms, giving \b[$\mathrm{C_{21}H_{19}OPS}$].
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