[GreenKeys] Nice old picture

[Jim Haynes]

As I recall there was also an adjustment procedure that involved holding
down the space bar and measuring the time to space across a line, as an
alternative to using the tuning fork.

And an aside to that, some military stuff allows for taking a bias
reading using the space bar held down.  This doesn't produce a zero
average signal, but the zero-center meter circuit is intentionally biased
to read zero when the bias is zero and the space bar is held down.