Having trouble embedding a full Facebook post but below is what the text says,
Our test today went perfectly. 3D-printed rocket fuel grain works like a charm, and we got data on how long it takes the flame to burn through an eight millimeter steel rod. Notice how it jumps about two seconds into the burn. This is going to find use as a reliable way to drop the early guidance system, so that the rocket can continue unencumbered towards space.
The really interesting bit starts about four minutes into the video. We are working on putting together a more streamlined video with more angles. Expect it soon.
-Kasper / RML
From what I can gather RML had the hybrid grain mould pieces 3D printed, whether these were removed or kept in the grain I am not sure.