Hard to tell what direction is up/forward. Inconvenient with the decoupler, but dangerous with the probe-core.

Added stencil lettering to the top/bottom of the probe core/decoupler to differentiate top and bottom, as well as note what direction is 'forward' (for probe-core use).

Could it be a different "top" color?

Would work for the decoupler, yes, but the probe core also needs to know what direction is 'front'. Which.... if I'm creative... could simply be a small triangle or something added to the front of the 'top' texture (so the sides are still smooth textured).