OMG, 2 days ago I finally sorted out the same problem. A leak where clutch line attached to bottom reservoir. First noticed little pool of fluid under front tire, rebuilt caliper, when bleeding noticed leak actually coming from bottom of connector and running down pipe and two different drip spots having made way to caliper. Loosened connector under reservoir with same internal turning that you saw. Outside the can on bottom, above the obvious fitting is another hidden nut, 11/16th that can be held and then wrench on connector will turn it while holding internal in place. No redoing of connector would work so I thought since internal tube would turn that system of washers etc. must be bad so replaced the reservoir, still leaked so only thing left is the shaped end on the clutch line must after 47 years hardened and will no longer take shape as fitting tightened. So replaced brake hose which by the way comes not preshaped so after much wrangling got it bent right and replaced and now all is well.
Jay
'65 3000