Igor

are you talking slings or sling swivels ?

Slings - don't know many who use slings on guns,
they might carry them in a pack which is always
a good thing to do.

Sling Swivels
Fitted to most English doubles.

And I just did a quick search on Drakes web site
and also gunsinternational, every Holland gun has
sling swivels attached, from BP Doubles onwards.