I believe "that lightsaber, it belongs to me" scene was left in the final cut which made things confusing. We now know the scene with the Knights of ren has changed meaning. At first we were told it was the Jedi academy students. And I strongly believe it's true I have other points to back it up. So somewhere down the line they decided to change it so the academy students turned into members of a clan.
If if you look kylo ren is holding something in his left hand, it looks like Luke's lightsaber. Many have speculated it's Luke's Rotj lightsaber. But if you hold the graflex at the right angle you can get a few rings of silver that look like the rings on the Rotj saber. Which is what everyone is pointing out
I strongly believe kylo stole the lightsaber from the academy in that scene. Some where down the line it was stolen back and given to Maz. There is a lot of continuity issues when it comes to the whole graflex story in TFA. Piecing all the rumors, and deleted/changed scenes together this is a theory I came up with
-kylo storms the academy
-kylo slaughters students
-kylo steals the graflex because it was his grand fathers
(maybe he steals the helmet too, remember early on we heard rumors kylo was collecting sith artifacts)
-some where the graflex is stolen or lost from kylo
-graflex ends up hidden with Maz
- when Finn ignites the graflex in front of kylo kylo recognizes it as his grand fathers " that lightsaber, it belongs to me!!"
this is a theory I pieced together and seriously think was on the story table at one time... Why things changed so much I don't know... I sure wish it went this way though
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here is the scene I'm referring too. even in this picture i can see bunny ears
http://i.imgur.com/kV8x5ZI.jpg