How many parts have you confirmed to be interchangeable between the SX-3 and SX-4? Between the SX-4 and Maxus Silver?
For myself, I confirmed the SX-4 and Maxus share the same charging handle, stock shims and stocks in general. The SX-4 didn't come with shims and I learned the hard way the SX-3 ones don't fit. I have also confirmed the SX-3 charging handle and stock shims won't fit the SX-4. The SX-3 charging handle was thicker and shorter and the SX-3 stock was also thicker where the shims went. SX-4 and Maxus also share the same aftermarket magazine extensions, but those probably fit on the SX-3 too.
There was a difference between the SX-4 charging handle and Maxus in where the spring loaded ball went. I had to slightly modify the Maxus handle so it clicked in properly but they shared the same profile and dimensions. The SX-3 handle wasn't the same profile and wouldn't go in at all.