The R-R line passes through a little bit of one edge of the EAFOTS sector. EAFOTS KB-B C27-11 is a system I have bookmarked that is right on the R-R line. There is a calculator here:
https://www.geogebra.org/m/KUhN3UtG (you have to read yor coordinates off the galaxy map).
I've checked every system within about 50 ly of the line that is within EAFOTS sector, and there's nothing there. There's also nothing special to see in the skybox/background either.
Beyond the EAFOTS sector the R-R line hits the Poseidon Wall. The "wall" is where the stars thin out. It starts right near the border of the HYPIAE and PHOI AESCS sectors on the R-R line. Before 2.1 Engineers there was a pretty big gap that you just couldn't cross without using 100% jump boost injections the whole way, so that's where CMDR Heisenberg charted a roundabout way to the outer arm that's now called the Heisenberg Bridge.
These days with a 50+ ly ship like an engineered Anaconda there is only about 600 ly of the R-R that is still not directly crossable, even with boosts. I have the gap bookmarked as being between PHOI AESCS V0-Z-D13-0 (on the edge of the Poseidon Wall) and PHOI AESCS DR-L C21-0. That's the real Formidine Rift that you still have to do a slight detour around. I've boosted into parts of it, and most of the G-class star systems have already been scanned by others. Nothing there.
The clues so far strongly suggest that whatever the mystery is, it lies *beyond* the rift. Since Drew has confirmed it isn't a USS or POI or a single system, and that it's been in-game since 1.0 gamma, then that means it can only be a pattern of stars and nebula visible in the skybox. To really see the skybox you have to fly about 2000ls out from the main star of the system, so that the skybox objects are fully brightened up (they are dimmed near the central star to simulate glare).
So, we're looking for a pattern that is visible in the background stars and nebula. What else could it be?
One problem is that the nebula don't always display - a known bug. So, that might make the mystery unsolvable just in itself, which would really suck. (And shame on Drew and FD is this is the case).
Another question is whether the pattern is visible *only* in the skybox, or whether it is visible in the galaxy map. If it's in the map, then no actual exploration is required. That would also be lousy - in which case, shame on Drew and FD again.
I'm back in the bubble now. I got frustrated with not enough good clues and the indications that the "mystery" can't actually be anything very interesting. I hope I'm wrong, but if you can't actually discover anything except that a few stars or nebula line up a certain way when you look at them from the outer arm, well, that's a rubbish "solution" to a 2-year-old "mystery" in my opinion.