The first set shown looks like stabilized burl? Either way both look almost perfect!
There's really only one way to prevent errors and to guarantee repeatability in regards to hole placement when undertaking manual drilling, and that's to take the time to layout and create a precision hole/drilling template of suitable (read: stable) material that utilizes hardened steel drilling guides. Or you could use 2D/3D modeling software to design both your holding/drilling fixture(s) and the grips themselves, and then create both utilizing a dedicated CNC router and the proper end mills. Perhaps you're already using the latter method to shape the grips since I see a nice tablesaw, moisture gauge, and a healthy supply of lumber also