Most of the pipe pieces use one of like 6 different incline angles. At these angles, there's always a corresponding piece that allows you to smoothly transition to/from horizontal.
E.g. Inclined Bend 118 has an incline of roughly 14 degrees, corresponding to the Inclined Straight 1234, and the Inclined Transitions 335 and 336.

I've looked at like every part, and they all use fairly universal angles.
That is, except for parts 121, 122, and 123.
These 3 Inclined Bends all use the same incline angle, which doesn't seem to match with any other part. It's around 20 degrees, I think, but there's no transition to perfectly match it.
It's somewhere between Incline Straights 113 (about 26.6 degrees) and 1234 (roughly 14 degrees, as previously mentioned). Both of these transitions work well enough, but they're just... not perfect. They don't quite look right.

Any ideas? Does anyone know what it pairs with (if anything)?