I have a 56 FS with an M10 L/H thread end. Please advise trimmer head type to fit this.

These heads would be correct for your machine: https://www.lsengineers.co.uk/non-genuine-trimmer-head-replaces-stihl-autocut-25-2-for-fs55-fs200.html https://www.lsengineers.co.uk/autocut-25-2-stihl-strimmer-head.html https://www.lsengineers.co.uk/autocut-c26-2-genuine-stihl-oem-no-4002-710-2169.html