Some thoughts:
Apparently, the M.2 drive's standoff is a 6-32 4mm standoff. Used to be a common PC standoff, before M.3 4mm standoffs became more common. The part number is printed in the silkscreen, 7W94R, but good luck sourcing it.
Seems to be a 3rd party screw kit on Amazon that has a 6-32 4mm standoff that converts to M3 threading up top.
The Extended's PCIe slot has a slightly undersized bracket opening and clearance for the card near the bracket area. Not a massive impedance, but some cards will run into minor (or major) fitment issues.
Aside:
Next time, I'm just going to eat the size cost of a mITX build (sheer cost as well). Still keeping these units around, since I do have uses for the serial ports. Less so for the parallel port, since I have microcontrollers for those kinds of projects nowadays. Shame the Lone L5 isn't around anymore, the Lazer3D HT5 is so expensive, the HDPlex H_ is expensive+unavailable, and the Streacom DB_ has the same problems as the latter. Clones are scarce, too. Must really be a dead end evolution. Not a massive fan of risers, but it seems I will have to go that way.