What's this bolt head type and how can I open it?
It looks like a spanner tamper-proof security bolt. Either buy a set of tamper-proof security bits, or modify a cheap standard slotted screwdriver with a dremel tool. Here is a cheap set: https://www.amazon.com/Performance-Tool-W8659-Security-32-Piece/dp/B002KS19PK/ A good chance that something in this set will work, but no guarantee.