The red plastic coupler that connects the lens housing with the flashlight body tends to break easily, so here's the replacement you need.
This is a replacement coupler for the Nebo Redline Tactical LED Flashlight
(Models: 5599, 5557, 5610 and possibly others, the red band is about 1/4" or 3mm wide - See Photos)
NOTE: This part does not fit the NEBO OC/NEBO Classic OC/NEBO Blast/NEBO Micro versions (these have a thinner red band, about 1mm wide, and the threads are wider)
Here is a version that fits those models:
This is the standard SMOOTH version, there is a new version with indentations around the ring:
In the photos, the blue coupler is the smooth version, and the red coupler is the indented version.
Note: The strong and flexible plastics have a variation of ± 0.15mm, then ± 0.15%. Since the threads have fine tolerances, sometimes parts are printed too small so that the threads won't engage. Please report these to Shapeways support and have them print you another, also please send me feedback, so I can make adjustments if needed.
Sorry, Turns out this version isn't printable using the Acrylic Plastics (thin walls).
Prints well using any of the Strong & Flexible Plastics.