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We use detailed radiative transfer models to investigate the differencesbetween the Star-Obvious Low-level-Elongated proto-planetary nebulae (SOLEPPNs) and DUst-Prominent Longitudinally-EXtended proto-planetary nebulae(DUPLEX PPNs) which are two subclasses of PPNs suggested by Ueta, Meixner, &Bobrowsky (2000). We select one SOLE PPN, HD 161796, and one DUPLEX PPN, IRAS17150$-$3224, both of which are well studied and representative of their PPNclasses. Using an axisymmetric dust shell radiative transfer code, we modelthese two sources in detail and constrain their mass-loss histories,inclination angles and dust composition. The physical parameters derived for HD161796 and IRAS 17150$-$3224 demonstrate that they are physically quitedifferent and that their observed differences cannot be attributed toinclination angle effects. If these calculations reflect a more general truthabout SOLE vs. DUPLEX PPNs, then these two subclasses of PPNs are physicallydistinct with the SOLE PPNs derived from low mass progenitors and DUPLEX PPNsderived from high mass progenitors.Comment: Accepted by ApJ. 8 fig

Two Subclasses of Proto–Planetary Nebulae: Model Calculations