@Accy cyclist you seem to ask us rather than Googling quite simple things (observation not criticism) so a quick overview of the SQR system I mentioned on p1.
A couple of my bikes SQR pics for illustration, with & without the Super C semi rigid bag in place.
The bag attaches to a small metal frame, The Super C bag is SQR specific so the frame is fitted so to speak inside the sheath on the top of the bag. The frame is removable and usable on its own, it is suitable for other Carradice bags too and emulates saddle loops so will happily take any saddlebag that a saddle will. You can see the metal frame then attaches and locks into the SQR block on the seatpost, gives a solid and rigid set up
View attachment 125856
View attachment 125857