From the professional trimmer section we find the Stihl FS 111 RX.While we already know the R in the model number stands for the rounded loop handle, the X at the end means it's a lightweight version.

Product Features. FS 55 R Trimmer Head Replaces Part Number: 25-2 Autocut, Stihl 4002 710 2191, 4002 710 2108 ;Trimmer head uses .080, .095, or .105 trimmer line.10mm x 1.0 LHF arbor size； ...

Other letters are often tagged onto the end of Stihl string trimmer model numbers, such as E, M, R, C, and X. One example comes from the homeowner string trimmer group, the Stihl FS 56 RC-E Trimmer .