here's some photos and measurements pertaining to seat and arm rest mounting rails

UPDATE!!! i found some flat square nuts that fit perfectly!!
they are 16mm square by either 3mm or 5mm thick (3mm pictured above)
and have an M6 1.0 thread pitch bolt hole in them
square nuts - https://amzn.to/41d91w6
M6x20mm hex cap bolts - https://amzn.to/499ZSGf
(amazon associate links)
you'll probably want to size them for whatever you need to attach though.. stacks of washers may be required depending on the bolt length
but 16x16mm square nuts are the trick!!
THEY MUST BE FLAT 16X16Xmm NUTS!! (up to 5mm thick) TRADITIONAL SQUARE NUTS WILL NOT FIT!!

ALSO..
SAE 3/8" HEX cap bolts seem to fit.. they DO freely rotate inside the rail, but they grip tightly regardless!
only half-height 3/8 nuts appear to fit into the rails
(if you need to mount something LARGE)
otherwise i'd use the square nuts above!

these measurements are approximate: hence the photos :^)
manufacturing tolerances on quantum chairs are all over the place..