Last but far from least, hovering near the SSD slots, we have two custom Apple chips:

Apple T2 339S00467 layered over SK Hynix H9HKNNNBRUMUVR-NLH LPDDR4

Apple 338S00268—this chip's a bit of a mystery. In our initial excitement we thought it was the rumored A10 Fusion coprocessor, first seen in the iPhone 7, but the package size is too small (roughly 7.4 mm each side). Best guess: this is an Apple/Dialog Semi power management IC.

The downside to all this added functionality and security is a major headache if your iMac Pro ever has to be restored.