Wow! Yours does look very mint! I'd love to keep mine atop the 301, but something about opening the lid is just nice (that's where he installed the motor kill switch).
The only information I ever found about it was in regards to where it fit in the timeline, such as a very brief history of Roland. There was some user based info on a forum somewhere with a lot of people still thinking it was digital. There is also a Japanese sales literature that I found during an image search which I included below.
Some of the people on the forums talked about adjusting the trimmers, but there's no telling what they do. One could only imagine it controls the self-oscillation of the hall echo.
If you watched the Chorus Comparison video, you will see that the DC-50 seemed to be faster and more intense, like a 90º sweep whereas the 301 had more of a 45º sweep. I don't know if Shane (Echofix) dialed down the chorus to a more musically manageable, guitar-friendly level. He also didn't have any info on the DC-50 and after I got the unit, I never could get back in touch with the Japanese seller.
Edit:
I took it apart again today and took some pictures in addition to the ones below snapped last year. It contains four trimmers, 2 MN3001 chips, a NEC C1458 op amp, as well as two TA7136p op amps.
Roland DC-50= ¥49,500/$239.34 (1978);¥73,990/$728.80 (today)
$239.34 (1978) = $898.20 (today)
Roland RE-301= ¥120,000/$501.38 (1978); ¥179,369.60/$1,766.79 (today)
$501.38 (1978) =1,881.58 (today)
Roland 1978 Catalog (Japanese) Vol. 3
http://brochures.yokochou.com/keyboard- ... index.html
http://brochures.yokochou.com/keyboard- ... /p3940.pdf
http://brochures.yokochou.com/keyboard- ... /p4142.pdf