Basically, those official prices are insane. You might want to consider budget alternatives so you have more cash for Japan. Make sure to look at these:

• JJC TA-M10 Aluminum Alloy Thumb Grip for Leica M10 is way cheaper and feels surprisingly sturdy.
• I would suggest checking for rubber padding to protect the paint.
• Be careful with generic brass ones tho, they can sometimes wiggle in the hot shoe.

Totally agree that those official prices are basically daylight robbery. Over the years, Ive learned that expensive name brands arent always the safest bet for your wallet. I tried the Fotasy Metal Thumb Grip for Leica M10 and it stays really snug. Saving that cash for your Japan trip is way smarter. Just make sure the fit is tight so it doesnt wiggle while youre in Tokyo. Lmk if you need other Japan gear tips!

I used the Haoge THB-M10B Thumb Grip for Leica M10 in London and it stayed snug. I would suggest it, but be careful cuz it still blocks the flash like the rest.

Seconded!