Jonas,
The steel gear/scroll I took out has very little mileage and is without any detectable clearance in the brass housing. The brass housing does have spiral grooves on the bearing surface which will be designed to react with the scroll on the steel shaft. Interestingly the spiral pattern in the brass is a double spiral, one clockwise and one anticlockwise. It might be a complete red herring, but does imply that the brass housing would be compatible with both LH and RH scrolls.
However I would not suppose that a worn brass housing would start to pump oil up the speedo. cable. All things considered I am rather stumped. To move oil up the cable and into the speedo. head must require one of three things: A pressure at the lower end, a pumping action or a head of fluid.
We can discount the last as the 'box is well below the speedo. The pumping action could only come from the scroll which, of course, is designed to push oil back into the 'box not up the cable, but would work in the wrong sense if the rotation or the scroll were reversed. As we see from the photographs the scrolls are identical. So far as rotation is concerned you say that the speedo. needle rotates in the normal manner to indicate speed. This could only happen if the cable is rotating in the correct direction - if the cable was running 'backwards' then the speedo. needle would stay firmly pressed against the little stop, on the dial, near zero. Which being the case only leaves us with pressure and you have incorporated a breather which must vent any pressure: Unless, internally, not all parts of the gearbox are connected and you will see that the speedo. drive, on the Moss box, comes from a separate module bolted to the rear of the main case. As the mainshaft comes right through this housing it seems unlikely that it is not internally vented to the main case and therefore to your breather. In addition there would need to be lubrication within this housing which looks as though it carries the output bearing and oilseal, so again you would assume internal venting.
Are you absolutely sure that the speedometer needle is working in the correct sense when you drive '31 ? If it is reading zero when driving forward does it read a speed when proceeding in reverse ?
Barrie