Unfortunately Chris's site doesn't cover the same model of Retinette 1A, and I could get a dozen for the cost of a repair job, plus postage to Aotearoa. I've been in this one from the front before, many years ago, so if no-one else has had a similar problem, I'll go in that way and hope. My suspicion as the shutter doesn't hang in any way at B, which I'd expect it to if it was oily blades, is that the linkage has somehow got deranged, though it's quite possible that it's gunge in a different escapement. My worry with that is that I could only get in so far from the front, and not at all from the back, so may not be able to reach the problem area. I guess I will have to find out the hard way!
Sorry Momus, it's a Retinette, not a Retina, so nothing as exciting as a Xenar, though you are right about the cramped access. My makeshift shutter wrench refused utterly to shift it last time I worked on it.
Adrian