How to open a Comptometer and get access to the mechanism is well documented in several places on the internet. The Supertotalizer is not much different, it starts with removing the front top panel by removing two screws (green circle). The panel can then be lifted off, moving it forward at the same time to slide the two prongs out of the side-beading (green arrow).
With some extra oil at likely spots and a thorough cleaning of the red correction-button stem, the controlled-key error detection mechanism also started to function as advertised.
The one thing that still won't work, is the main bell. The mechanism is there and seems correct, the rock-frame however just doesn't put enough 'zest' into its engaging-movement to give the hammer enough momentum to strike the bell. We can live without a bell.
The Felt & Tarrant Supertotalizer has now been oiled and continues to be extensively exercised - and all the main functions work again.
Now discovering how to operate it; a fascinating calculator and it's surprisingly fast and capable.
Typewriters are simple :-)