As noted by several people in the 'Typosphere', the Noiseless Portable is prone to skipping. It needs a very staccato typing technique to work well. Any hint of 'bottom-typing' (pressing the key right to the bottom of its travel) will cause random skipping. The skipping is not completely random though, especially the letter 'a' is very likely to cause a skip.
Looking again at this very ornamental, pleasantly quiet, but still-skipping machine; noted that the letter 'a' is right towards the end of the universal bar triggering the escapement. Over time, wear of the whole mechanism could well mean that the bar is out of adjustment, worn or bent even. A possible cause then could be that the keys trigger the escapement a bit too late, especially the keys at the left-end of the bar.
To test the hypothesis, placed a thickness of some layers of tape on the universal bar right where the letter 'a' strikes it. Uncanny the difference that made. No amount of bottom-typing would cause the machine to skip! Removing the tape again it was up to its usual playful skipping routine. That small test seemed to confirm that setting the escapement trigger a little earlier could do the trick.
Then looking for the adjustment screw - there had to be one.
And there is - it is a small screw-pillar with a lock-nut. The whole linkage that pushes the escapement is a bit 'loose' by now, can well imagine that the play caused by the 8 decades of wear make an adjustment necessary.
A turn clockwise (in) of the pillar seems to have done the trick now.
The very ornamental Noiseless machine should no longer skip right at the end of an otherwise error-free page :)