  We use detailed radiative transfer models to investigate the differences
between the Star-Obvious Low-level-Elongated proto-planetary nebulae (SOLE
PPNs) 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, IRAS
17150$-$3224, both of which are well studied and representative of their PPN
classes. Using an axisymmetric dust shell radiative transfer code, we model
these two sources in detail and constrain their mass-loss histories,
inclination angles and dust composition. The physical parameters derived for HD
161796 and IRAS 17150$-$3224 demonstrate that they are physically quite
different and that their observed differences cannot be attributed to
inclination angle effects. If these calculations reflect a more general truth
about SOLE vs. DUPLEX PPNs, then these two subclasses of PPNs are physically
distinct with the SOLE PPNs derived from low mass progenitors and DUPLEX PPNs
derived from high mass progenitors.
