Hi!
Recently I sent a few 1000 tunas for batt replacement. Three tunas that use allen/hex screws.
I'll pass along the experience I ran into with them...
My watchmaker had some tuff times to extract the screws without damage. In fact he wasn't able to.

After learning from the mistake of using an allen wrench of the flat head type, I got a solid advice from my friend Shannon Spence about tuna allen screws and the correct key type. In fact, Shannon's advice is worth its weight in gold!
Shannon tipped me off about the Bondhus allen screwdriver. Bondhus is the company that actually created the allen/hex screw, to begin with. So they know a thing or two about how to extract them without leaving any marks, no matter how tight they are (and the
[email protected]$&*ers at Seiko know how to attach them tight!



).
The Bondhus screwdriver has a ball, espheric head, so when one inserts it into the screw, the Bondhus tool gets snugly, juxtaposed fit inside the screw, so it allows for a great force transference into the screw upon extracting motion, without any spaces or play inside of it. And the fact that it has a ball end/head allows for a proper fitting inside the screw, thus preventing striping or marking the screw head.
The allen screws size used in the tunas (both SBBN015, SBBN017, SBBN011 and SBBN013, and also SBDX011) is exactly 1.5mm. A 1/16'' screwdriver won't fit inside the screw. Ask me how I know!
I've bought both sizes of the Bondhus screwdriver: 1.5mm and 1/16''. Only the 1.5mm fit inside the screw, but it was a 100% match, perfect fit of the exact type. The 1/16'' screwdriver didn't allow to reach full inside the screw head, where the ball end should sit snugly to allow proper torque motion in a way not to damage the screw.
The best part: you can buy a pair of Bondhus ball screwdrivers of 1.5mm on ebay, from a seller called omnitools (they sell stuff for aeromodelism), for around $4 usd a pair!
Not affiliated with the seller. Just spreading the news.
I can't thank my buddy Shannon enough for such golden tip as this.
