I was hoping it was the same. I just bought a baby Arnie that the analog function was dead. I had a complete good H556 in my inventory so I grabbed it. I noticed my spare H556 has the stem and crown in the little baggie with it so I knew I had an extra and I was hoping to solve your problem but I could see from the repair manuals they are two different animals. It looks like the H558 stem is actually two pieces that thread together from the parts lay out but I could be wrong. Hopefully someone has one lying around.
Man that sucks! The stem does have that weird end. It has those two paddles that it uses from the digital adjustments that stick out. Maybe they did not get lined up when the stem was inserted? At least you can replace the stem with another $25 bucks. Imagine if it was a part that was discontinued that just snapped. Now that would really suck!
Good job - finally!
The hands look fine but...the bezel insert could do with replacing.
I replaced one for a friend. I found one on ebay(a seller with various bezel inserts) that, while not original was the right size and looked the part