Questions about Stihl Thicket knife

Is the blade compatible with the Stihl Fs 55r?

Good day. I have a 20-year-old back scythe from Stihl. In addition to an AutoCut 2-2 mowing head, a blade is also fitted to this brushcutter, although this blade has a relatively small diameter (no longer commercially available). The back scythe is mainly used for mowing a steep slope. The grass there is tough and bushy, which is why I am considering changing to a thicket blade. On sale you now have the above-mentioned brush cutter 350mm. and the brush cutter 250 for grass trimmers. Can I fit the 350 mm brush cutter to the back cutter (2-stroke) without hesitation? The diameter of the cutting circle (guard) is large enough to fit the 350 mm brush cutter. I look forward to your comments, thank you very much in advance and remain with kind regards: Beat Luginbühl