To fit flex mounts in to those bricks you need the right mounts. As the usual mounts associated with being called 'flex mounts' are actually too big.
Here is a picture of the inside of a brick that came with brass flex mounts
Attachment 43094
And some close ups (the small squares are 1mm)
Attachment 43095
Attachment 43096
Attachment 43097
I don't know where to get these mounts, but if someone out there does, I'd be interested to know as I have another brick with the round ally mounts I would love to change.