here's some photos and measurements pertaining to seat and arm rest mounting rails
(hint - they're all the SAME!)

Metric M6 HEX cap bolts & nuts fit nicely into the side, and arm rails
likewise M6 square nuts fit like a glove!
ALL hardware was purchased from ACE Hardware

the seat side rails have a bit of a + 'plus' shape to them
and the armrest rails are more of a standard rectangular 'U" channel
(see photos)

these measurements are approximate: hence the photos :^)

BTW the rails are spaced 1" apart from each other