Just checked the schematic at the ETF and it looks like it's coil B4, it's mounted in a can on the top front of the chassis next to the Horiz Osc tube, a minor adjustment of that coil should bring it back to center.
There's also the possibility that the Osc tube is a bit weak and doesn't run well when it's cold but often you will have to readjust this coil when the tube is replaced anyhow.
Schematic is here:
http://www.earlytelevision.org/pdf/P...Sams-115-8.pdf