From having bought the parts of a 'boat-anchor' Remington Portable typewriter, there's a good supply of screws and sundry bits 'n bobs in the spares box. With the benefit of reference machines, two of these bits have now been identified as the spring to press down the margins-bar and its mounting screw.
Comparing one typewriter with a next, it turned out that my 'statistical machine' did not have this spring fitted. It may never have had it, or may well have lost it during its 90+ years of alternating use and neglect. In any case, to be fitted with one again. The Remington Portable still works fine also without this part, the weight of the margins-bar is enough to make it drop back in the correct position after any 'margin release' use. With the spring it'll work better and more reliable, it should then even be possible to type with the typewriter held upside down (?).
The spring is placed with the long end tucked into the corner of the margins-bar. The eyelet shown in front of the threaded hole for the mounting screw.
With the mounting screw through the eyelet of the spring, it is held in place and the margins-bar is kept in position more firmly.
(Whilst not strictly necessary, it's a lot easier to mount when the platen is first removed. On the Portable this is a fairly easy procedure.)
Everything re-assembled - hardly noticeable, yet a more complete and fully functional Remington Portable typewriter.