Greetings All,
I just purchased a galil ARM, and I noticed that the bolt carrier has a lightning cut in it like the newer ace's have. It works smoothly, but I feel like it may not be original. Can anyone verify if any of the original Galil ARMs had a lightening cut in the bolt carrier? Here is a picture:

you have an early MAR bolt carrier with an AR/ARM piston.
the lightening cut and the turned down step on the front is straight up MAR.
the step is so the piston can go all the way into a MAR gas block.

I'm guessing the kit you have is from blackthorne- they used these when they were running out of parts.

Thanks! I didn't actually get this from a kit, but bought the gun already built. The question now is, would this work without any issues? Also, I am having the barrel replaced (professionally by two river arms, timed, etc), and I am getting a full ARM parts kit. Would you see any reason why I couln't change out the new bolt and carrier from the parts kit with this old one? Headspace won't be an issue since they are going to do that when they put the new barrel on anyway. Thanks!

bolt carrier should work fine.
as long as the new bolt is seated properly there shouldn't be any issues.

If I'm not mistaken, isn't Blackthorne one of those alias names that Hesse Arms used besides Vulcan, to hide the fact that they are producers of questionable products. They seemed to produce rifles from parts kits that tended to be out of spec like Century Arms.