That is the current spec'd after SB 3 came out in April 1962. Prior to that, it was 130mA key down, with no excitation. The comment on the reason for the SB from Collins was to lower plate dissipation.... No kidding!
I needed new tubes in mine when i bought it, so it received 4, 572b's. They have much more plate dissipation capability.
My Dentron Clipperton QRO (replaced Clipperton L) idles at 70ma in CW/1900V setting and 120ma in SSB/2700V setting.