Parallel Design

The SKS and FAL both use a tilting bolt and a short stroke gas piston.

The SKS has a two-piece piston, the FAL is one piece.

The bolts differ a great deal in detail and you can see that while the principle is the same, the designs are not inspired by or derived from the same source.

Bolt and carrier in the locked position.

Bolt and carrier in the unlocked position.

Bolt all by itself.

In both cases the bolt hits the end of the barrel and the carrier shoves the rear of the bolt down into the locking shoulder.  On the SKS the rear of the bolt has a ramp that acts on the rear of the protruding "hook" on the bolt.  On the FAL the top-rear of the bolt carrier has the ramp that shoves on the top of the back-end of the bolt.

For unlocking, the SKS carrier grabs the "hook" with a rear facing ramp and pulls the rear of the bolt up.  The FAL has a lug on either side of the rear of the bolt that are lifted by rails on the interior sides of the bolt-carrier.

Image from: http://www.gundigest.com/wp-content/uploads/2011/05/FN49-3.jpg

Although this is from the top, not the side, you can see the SAFN has the same two-lug rear to the bolt that rides on rails in the bolt carrier.

Image from: http://www.gunpics.net/russian/svt40/svt4022.JPG

The SVT-40 is kind of a mix of both designs, which is odd since it precedes the SKS and is contemporaneous with the development of the SAFN!  The SVT uses rhomboid shaped lugs on the rear sides of the bolt that are engaged by a matching recess in the rear of the carrier.  But the same lugs are used for engagement and disengagement, just like the SKS.  Edit to add:  It's also a Tokarev design, not Simonov.  Lots of people get the SVT confused with the Simonov AVS-36 when talking about the design and function.  The AVS-36 uses a vertically sliding locking block, much like a Winchester lever action of all things, rather than the tilting block of the SVT-38.  The confusion might stem from Simonov's more successful designs both being tilting.

The FAL and SAFN use separate surfaces for each operation.

Mind numbingly interesting, no?

The hook lug on the SKS actually more resembles the tilting bolt of the MP.44 than something derived from the SVT.  Much like Comrade Simonov copied the bolt and adapted his SVT carrier and gas system to it.

Image from: http://www.gotavapen.se/gota/ak/stg/mp44_bolt.jpg

  Except the SKS bolt and carrier have far too much resemblance to the PTRS-41...  The SVT appears to be the outlier here.

On top of it all, Msr Saive and Comrade Simonov weren't in communication with one another, even indirectly.  Their designs sprang from their own selves and weren't cribbed from the work of others except in the broadest strokes.