single-digit locks for shifting

One of the subtleties of locking that Babbage doesn't seem to have detailed anywhere is what happens at the top and bottom of the digit stacks when shifts (which multiply or divide by 10) are done. If all digits are unlocked in the normal fashion, then the least significant digit (for left shifts) or the most significant digit (for right shifts) are undriven and are subject to accidental movement. 

The solution is to devise a mechanism that will lock only that one digit. Here's an illustration for number stacks that are four digits high, showing which digits need to be locked.

In my continuing quest to minimize the number of vertical axes, I came up with a way to do that with the existing long pinion locks: a single extra locking lever that is engaged by a "backwards" rotation of the axis. 

When fully rotated counterclockwise, all locks are engaged with the inner gears of all digits. When fully rotated clockwise for shifts, only the one "backward" lock is engaged with the outer gear of the topmost or bottommost single digit. In the in-between position for normal transfers, all locks are disengaged.

It seems to work well, and now zeros are correctly retained as the least-significant digit for left shifts, and as the most-significant digit for right shifts.