What is this bolt called?
Searching for "Large head socket screw" gets close, but no cigar.
Also try 'cap screw'.

You need to know the thread, of course.

You may be able to replace it with a (rounded) socket button screw, eg. https://www.kayfast.co.uk/category/517/Socket-Screws

Update: Search Socket Pipe Plug, also described as Flange Plug and Blanking Plug - these look very similar:
https://www.accu.co.uk/en/flat-head-socket-pipe-plugs/177868-HPPF-M10-1-A4-BL

Or: Slotted blanking screw eg. https://www.ebay.co.uk/itm/NiceCNC-Billet-Mirror-Hole-Blanking-Plugs-M10x1-25-Aluminum-Motorcycle-Universal-/303185376819 - interesting that these are also available with a left-hand thread.

You need not (necessarily) be wedded to the allen socket - an ordinary M10 bolt over the end, perhaps with a suitable washer, might suffice, for quick replacement.