Anyone able to identify this screw head?

One of my Dura ace 9150 rear mech's is on it's way out.
Shifting down is fine, but back up under the strength of the servo is tough.

I've ordered a replacement mech, but would like to (out of curiosity) pull apart this one and see what is going on.

But, for the life of me, I cannot find out what this screw head is.

I've spent many hours on google, even reverse image search failed me.

A Shimano proprietary tool that only they have?

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If you just want to unscrew it, would a tri wing screwdriver not work?

It does seem the closest of things I've seen to maybe work.

Surprisingly a flat blade does engage, but more torque require than I'm willing to put into it at the risk of stripping the head

Put a wide elastic band in first for extra purchase.

Hammer in a torx screw driver and undo it. Bosh!

You could try grinding a Tri-Wing bit to fit

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Fill it with bicarb, put in a bit which nearly fits, and add superglue.

If you want to take it apart you'll need two things, possibly more if you wanna put it back together:

-A set of tri-wing security bits, I bought this one
-Heat. These bolts are installed with red locktite, and you'll need to heat them up proper before trying to unscrew.

@Di2_GP was kind enough to guide me through fixing my own, so paying it forward. I used a heat gun to heat up the screw (and placed tinfoil around it to avoid damage to other bits), and it took a good amount of heat to get it to move.

If you message him on insta he might be able to help you too, and if you dont end up fixing yours please consider donating your broken one for parts. Or he might even be able to fix it!

If you end up fixing it, this is what you'll need to put it together:
-M2.5 x 10 torx head screw
-M2.5 x 6 torx head screw
-Loctite 243

These are so that you can take it apart easier the next time if you want to. The lengths were for my RD-r8050, yours might be different. Check out Pawel's insta with very useful videos: https://www.instagram.com/di2_gp/

This is exactly what I was hoping for.

Amazing. Thank you!!

I have a new to me (used) mech on there now.
So in no rush, but as I have 2 bikes with this rear mech, it'd be useful to have a working spare

Happy fixing!