The lightsaber hilt used in TFA was closest to the ESB one(s). The Replica Prop forums guys have discussed it since seeing it in the the trailer back in April. They are very OCD about this stuff, and they were happy that it was a graflex or very close replica much like ESB was used. TFA one actually looks more like ESB than it does ROTS. There are differences in the props even in the context of the same films. You just don't see close-ups very often. So don't even go there.
Most of the time people turn the saber on with the rectangular box panel near the grips. But it could be possible that the higher button could be a duplicate activation switch that Finn uses, it is stated that Rey uses it too, but I've seen the film 8 times now and she activates it with the box near the grip like usually done. The knob part around it serves as "Blade Length Adjust" as shown in the Visual Dictionary. Pablo Hidalgo https://twitter.com/pablohidalgo
is the head of Lucasfilm Story Group/Creative Executive and he writes the Visual Dictionaries and such. He also advised JJ on the film, as JJ stated in interviews including this one. http://www.thereporter.com/article/zz/2 ... /151206558
This following link is the saber from TFA, though the scene it's shown in here was cut. http://www.telegraph.co.uk/content/dam/ ... xlarge.jpg
and here at the top of the cover of the TFA Visual Dictionary. http://vignette1.wikia.nocookie.net/sta ... 0926003658