This is only a matter of semantics, I'm sure, but I've always considered a dolls head, and hidden rib extention, to be a "THIRD FASTENER" with the two under bites, being numbers one, and two! The screwgrip,and the Greener corss bolt, to me, being the fourth fastener!

The screw grip is most usually coupled with a doll's head, haveing a bite in the nose of the doll's head, for an angled top lever extention to engage. The angle pulling the doll's head down as the lever goes back to center after closeing the rifle. If one examines the greener cross bolt closely, you will find the cross bolt doesn't engage the rib extention from a 90 deg angle to the extention, but engages the hole at a shallow angle, working the same way as the screw grip, by the silghtly tapered pin pulling the barrels down as it slides home.

It is true, however, that not all greener cross bolting is properly done, as is the case with cheaply made screwgrips. An example of this is an old MIssissippi Arms 12 ga I own, is equiped with a very sloppy screw grip, that barely engages the extention at all.

As far as a double rifle needing a third, and/or fourth fastener, I believe the more lock-up one has the better. That, however, is in no way saying, a double rifle without a fastener of any kind other than the under bites, is automaticlly a weak rifle. That depends on many things, not the least of which is the chambering!

I agree with Mark, in that most shotgun conversions can benifite from a third, and fourth fastener. Haveing said all the above, most "off face" condition found in a double rifle can usually be traced back to poor care, more than lack of lock-up. The biggest offender is the guy who never lubes the hook/ pin mateing surfaces, or lets them rust, dirrectly causeing the looseness, more than the type of fastening in the rifle, or pressure.

Just my 0.02 cents!