Thanks a lot for the input.
Can this kind of aperture be folded down, collapsed or taken off when the scope is in use ? I read on XS-Sights' comment it needs drill and tapering to be installed, unlike their other ghost ring back-ups.
I found another kind of attachment aperture at NECG and Recknagel too. It fits the rear scope mount just like a ruger ring.
A folding sight, always ready to be used on the gun would be favored but if I can't find a way to have it smithed will consider attachments.
BTW I read Jeff Cooper, I have to confess it
He seems to advocate the use of a post front sight wiht a ghost ring. Would I have a problem to use a ghost ring with a ruger bead front sight ?
Lots of questions here, sorry, but, except for the military stuff, aperture sights are far not common features on rifles this side of the pond.