[Amps] grid bias supply

[Steve Flood]

I was looking at an amp design from a 1959 issue of GE Ham News.  Two 813s grounded-grid linear.  What makes this design different from the others I've looked at is the addition of an adjustable grid bias supply.   What functionality does this feature add?  From what I could glean from the text, it is used to set the zero-signal plate current to 40 mA for SSB tuneup, and for "almost zero" plate current for CW tuneup. Is this amp trying to operate in two different classes?

Steve KK7UV