As I understand it, the pedigree says that

P-17 was a seedling from a cross between imbricata as pod parent and
(M3) from timofejewii x reichenbachii as pollen parent.

The resulting seedling (P-17) was then crossed back to pollen from the
seedling M3, resulting in Sherman (sdlg AA-24-5)

<P-17: (I. imbricata x M-3: (I. timofejewii x I. reichenbachii "Van
Nes")) X M-3. >