mac pro 2012 pcie slots
Sonnet M.2 8x4 Silent Gen4 PCIe Card
On a 2008 Mac Pro (a.k.a. MacPro3,1), it apparently has 2 PCIe 2.0 slots and 2 PCIe 1.1 slots. While the difference won't prevent any PCIe device from working, it does make a difference for speed.
Pancake (slot car)
Create model railroad and slot car layouts on the Mac: Say Hello to RailModeller Pro. Available on the Mac App Store.
kubet ful - Solar Hà Giang
PCIE x 8 Dual U.2 to PCIe 4.0 X8 Adapter without PCIe Bifurcation Function 0.15 Kilograms PC/Server (PCIe Slot support PCIe Bifurcation: X4X4 or Hyper M.2 X16 mode), Not support Mac Pro.