unless you are very friendly with a Stihl dealer then you can not get your hands on a repair manual.
Stihl are very protective of their IP and usually any one who tries to sell Stihl manuals get a little visitation from Stihl's enforcers.
Some of the European supplies have parts breakdowns that are view only and that is as good as it gets.
I have a 2000 download of their dealer net manuals and the person I got them from was prosecuted.
Any web forum that hosts Stihl repair manuals is closed down or forced to remove it.
While I feel it is a bit over zealous, we have to remember that most Stihl products are potentially very dangerous so not encouraging Joe Public to pull them apart is not a bad idea.
And if you are talking about the finger guard which doubles as a guide rail, then my point is made as it is an essential part.
However they can not stop a tech doing a video so that is your best bet.
Some of the video makers actually are Stihl dealers and are making good money from their video channels so you might try asking them how to fix your problem.
The number 56 is the code of the power head so an FS 56 is the same engine on a line trimer thus if you are having an engine problem you can look at any 56 repair instructions.