Dual drive, drive

So new to me way for a print to fail.
Running overture fast TPU, half a roll left, good stuff, made toy car wheels etc.

thought I had a clog, but quickly clear a feed issue.

now if you unscrew the tension knob all the way, the lever arm can drop allowing on to peer into the depths.
mine had a bit of blue petg, some yellow pla, and an inch or so of TPU.

Canned air is great, Yeah?

So, my question. of the 4 tapped grub screw holes on the gears, how many should have grub screws in them??