I recently got my old Apple II Plus going with help from a member with a spare MM5740AAE/N.
Anyway, I noticed an odd thing with the color killer circuit.
It is "on" when booted (black/white only).
You can run something in color, like just "GR" from BASIC. Color looks fine.
Try to turn color off, like "TEXT" from BASIC. Color remains on.
Color mode won't turn off until I power the machine back off. The color killer is never re-engaged.
EDIT: I think this may be a tuning issue between my monitor (Sony PVM) and this early board (Rev 4). My other Apple II+ will switch this monitor in and out, but not this one. I can power off the display and back on, and it will be black and white again. So the CK is working, just not "enough".