The cuts in the handguard are for the retention spring that impinges on the front of the receiver/trunnion. They're not from the real gun I would imagine, but they are real wood and patterned correctly. The "nipple" on the bottom of the receiver is a rivet.
If you're using 2/3A cells I imagine you could stuff a 12V in there no problem, though I'd question why you'd want to. If anything I can see someone making a longer battery out of 4/5A or A cells to achieve higher capacity but for the extra expense of those cells, you're probably better off buying two Elite 1500s in whatever voltage you require and stuff the second into your gear. I'd recommend a 9.6V battery for these guns as the 8.4V may leave some of you wanting in the ROF department, even with a rewire and Deans connectors. Go to a lighter spring though, and you should stick with the 8.4V if you want a realistic ROF.