M900 piston circlip tool - after spending about 2 hours failing to get the circlips in I knocked this up from 20 mm aluminium bar - the circlip end engages on the grub screw and it took less than 5 minutes to install all circlips.
Just made this tool to help fitting belts on my 749: I stuck it in the centre/drive pulley to stop the belt disengaging while doing the tricky vertical cylinder belt. Picture shows it when doing the horizontal belt (which is much easier as both valves are closed). Sorry, I have no dimensions or material specs to pass on.