The mouse is held together by 5 small screws. It's a lot easier to open up compared to the G502, because there are no hidden clamps. Once you pry it open, you're going to notice everything is attached to the main PCB. The interior build looks pretty, but it's not on par to that Proteus Core.
Yes, it seems Logitech has changed the insides a wee bit. The brain and heart is a 32Bit ARM MCU, the STM32F102, a 48Mhz controller with 32 Kbytes of Flash Memory and 6 Kbytes of SRAM. It uses a variant of the AM010 sensor, able to switch from 240DPI to 4000DPI, in 80DPI increments.
When it comes to switches, Logitech's G402 is not necessarily the best option available. The left & right click are the OMRON D2FC-F-FN rated at 20 million clicks, but any other switch is Himake White, rated at only 300k clicks. I heard that there was a TTC version out there, but the sample we received contained these switches. They do feel slightly mushy, even though they have an OMRON like audible click.
Mousewheel itself is not really all that impressive either, though it is considerably smaller and lighter than the one on G502.