Regarding the 175 - this was originally created to work with a 1.75mm conversion kit I sell (I give these away with the kit) because I realized many people put this in the second nozzle position on the head and then just switch the bowden back and forth to switch filament. But this part works fine for either 1.75mm or 3mm filament - it makes no difference as the teflon part and the outer diameter of the bowden are the same.

oh and your file names say 1.75 but looking at the dimensions it seems for the standard 2.85 setup.

sounds like a smart idea, I'll give it a try next time I need to take the head apart.

3 reasons for the hole - all to do with vision:

1) You can see if the bowden is properly seated - it's possible the that bowden feels like it's all the way inserted but it's hung up on the aluminum above.

2) You can see the filament - sometimes I think the filament has hit the nozzle but actually only just got past that hole. this is not as important.

3) Proper assembly. Tighten/rotate the round not all the way until it won't go any further, then put it all together except for the bowden. Then look inside that hole as you "loosen" the round nut (or raise the nozzle) and you can see exactly when the teflon touches the ledge inside my part. That way you don't put any extra pressure on the teflon part.

I printed korneels original version in XT, used it for a long time, temperature is no issue. Any reason you added the hole in the front?

https://www.youmagine.com/designs/ultimaker-2-spring-replacement