Actually, that would be a 'stemmed seeded apple', as Stiel means something like stem or stalk. Sound notwithstanding, it has nothing to do with steel, which of course is Stahl in German, as in Stahlhelm. The Stielhandgranate 24 had a handle, as opposed to the Eihandgranate 39 (egg grenade)
If your grandfather is still around, as someone whose grandparents are all dead, I urge you if you have any desire to know more, to gird your loins and ask him. There was a lot of stuff I was too chicken to ask my grandparents, and now they're gone and I could kick myself for not asking.
Then again, it's also a good thing to respect his wishes, and if you ask and he says "no," that's another matter.
WWII. He was an artilleryman all over Europe, which is all he's ever really told me about it. Everything else I've had to infer from the war junk box. Looking up this word made me realize what that thing really was.
I've learned potato masher-as-grenade since then, but at the time I'd had plenty of experience with an actual wooden-handled potato masher (thanks to grandma). I don't think I wondered what a potato masher was doing in a box with medals, patches, knives, and such. Grandpa is an old pro at deflecting questions about the war, so I'm sure that's how he wanted it.
Grenade comes from the same root as pomegranate, which means you could translate this as "steel-seeded apple," which is something Byrhtnoth could have used against the Vikings.
When I was nine and looking through some of my grandfather's "war junk," I held up a stick and asked what it was. "Handle from a potato masher." Boring enough that I never thought about it again. He had a bad war.