[Reposted with repaired links]

I thought I had corrected this but I had not and I humbly ask for help.

Overall view of the back of the IBM model B Electric:
Side by side comparison of left margin on successive paragraphs:
In the first case the rear carriage cover was on and in the second it was off. I did nothing else but take off the cover. The seeming fix merely by removing the cover is reproducible but the issue comes back when the cover is replaced.

The margin stops slide on the bar, the left margin has a brake to cushion the return.

Moment of strike:
Brake lever has been actuated, and extreme travel is one space beyond margin.

Relaxed just after strike to the correct margin position:

I cannot figure out what effect removing the cover could have, as it would seem to have adequate clearance for the mechanism and be incapable of interfering with it. The margin sometimes wanders a space to the left and sometimes a space to the right but works correctly indefinitely with the cover removed. I've cleaned the visible parts. What else can I do beside always typing with an exposed carriage?

I've been thinking the same thing.  I find the margin problems that I have run into came mainly from the "Spring-Set" variety--as you find on Royals, Remington KMCs, later L. C. Smiths, Smith-Corona 88 Standards, and Hermes 3000s (the only Hermes I've run into besides the 808, a "Selectric" variant of the Hermes brand, but that one has "Slide-Set" margins).  Anyway, with the spring-set, old gum and dirt will cause the most trouble, along with the occasional stretched or detached spring.  Slide-set type margins, found on most typewriters, are pretty elementary--just push down on the margin button, and slide it to where you want the carriage to stop on either the left or the right side.  Again, dirt and gum present the most trouble.  The worst I've run into in the slide-sets are Underwood old-style machines--especially the right margin stop, which sets the left-hand side of the carriage.  These suckers don't just stick--they FREEZE.  A bit of hot tranny fluid or penetrating oil and a little elbow grease are about all that's needed to unstick these.  And then there's a third category:  "Hook-On" margins.  Some early Remington Super-Riters, IBM type bar models, and Underwood electrics use these.  Essentially, all they are are slide-set margins with a bit of extra linkages so you can press a key on the keyboard (or a little above it), and press down on either the left or the right hand margin, depending on where the carriage is oriented.  All the slide-set problems, plus any sticking or bent conditions in the levers are the main problems here. 
Now there could be some other mechanisms out there I don't know about.  These are all the ones I've run into, so this little post isn't all-inclusive.  Please let me know if anyone has a different margin mechanism, what problems they've encountered with those, or different problems with any of the three that I've outlined.

TypewriterKing wrote:

... Slide-set type margins, found on most typewriters, are pretty elementary--just push down on the margin button, and slide it to where you want the carriage to stop on either the left or the right side. Again, dirt and gum present the most trouble. The worst I've run into in the slide-sets are Underwood old-style machines--especially the right margin stop, which sets the left-hand side of the carriage. These suckers don't just stick--they FREEZE. A bit of hot tranny fluid or penetrating oil and a little elbow grease are about all that's needed to unstick these..

In my very limited experience I had already run into that. Twas an Underwood #3 (?) bought in the neighborhood for $25 and stop had likely been frozen to the far right a very long time. I got all apart so all I had was the rod and the slide and a collar also frozen to the rod next to the slide. I tried penetrating oil and more penetrating oil and drilling a hole in a piece of wood so I could insert the rod and tap while the wood held the slide and gentle heat and yet more penetrating oil... more ticklish because if I knocked the slide into the collar and it was still jammed it would have been worse.

I bought one of those tiny butane torches and filed down the rod between slide and collar (could not use my wood jig to knock it the other way) to bright metal so if it moved it would definitely be free. If it were a movie and I were trying to save the planet from exploding the timing could not have been more artful - I was just about to give up when the slide moved a fraction of a mm ! The planet was saved.

After that I lost interest in the project and popped a can of Pringles because it was too darn hard.  But I freed the margin stop. Recalcitrant it was.