Does anyone know how to tighten the rail/focus lock (the tab/lever near the rails that can be turned 90 degrees to keep the focus from moving)? I tried tightening the screw, but it didn't seem to make much difference when it came to the rails.

Remove the left yoke guide, lift the yoke out of the bed, then restart the yoke lock lever at a different position is one possibility. The other is to remove the right yoke guide also, then add or replace the brass yoke lock pad.
https://graflex.org/manuals/45-Pacemaker-Speed-and-Crown-Graphic.pdf
pdf page 20.

Success! Thanks for your help.

In case anyone comes across this thread later, make sure you apply tension on the brass strip springs by pressing down using the focus rollers (while simultaneously closing the bed/door part way). Was tearing out my hair trying to get the yoke guides back together before I realised what to do.