Great detective work.
Let's collaborate on this for the benefit of others who may have the same problem.
L.IN should be the point at which the Lead-In groove on the LP would be, where the tonearm will drop to play an LP
RET should be the point at which the tonearm will lift up and RETurn to its rest at the end of the LP
That leaves us with
SW. If your source is right this will affect the
Speed. I can't imagine what the W stands for.
Since this turntable lacks the traditional strobe light and dots along the platter rim for determining if it's on target speed-wise we have to improvise and make a Strobe Disc.
It has radial marks on it and you place it on the turntable like a record. The lights in your room running at 60hz will make it appear to be stationary if the speed is correct; slowly creeping CW if too fast;
CCW if too slow.
Here's a link to a
PDF from which you can print your own disc...
http://www.extremephono.com/ftp/60Hz.PDF
Here's what you do. Lay the trimmed Strobe Disc on top of a record while it's playing one of the outer tracks. This is because the stylus imparts some drag so we want to factor it in.
Then adjust the screw exactly one turn Clockwise and note how the speed has changed. Trial and error should get you to where you want to be.