here's some photos and measurements pertaining to the seat mounting rails
(this is the only invacare chair i have to measure)

Metric M10 HEX cap bolts fit into the siderails
they do rotate somewhat freely, but have plenty of clamping force!
M10 square nuts seem to be a better option than the M10 hex nuts
(see photos)
ALL hardware was purchased from ACE Hardware

these measurements are approximate: hence the photos :^)