There's an interesting and informative discussion below about the technical differences between IWC 8541 and 8541B movements. Because it involves very technical issues, I thought some background information might interest some readers.
A balance has a spring (a hairspring) that attaches to a balance staff. The "collet" is the part that surrounds the balance staff and the hairspring in affixed to it in the middle. The above illustration shows a modern balance (which by the way, is not an iWC) and you can see the collet, the balance staff and the hairspring.
For IWC's in-house automatic calibers 852x, 853x and 854x, IWC used a collet of its own construction. Then in the 1970s IWC started using a regulation-machine made by "Greiner". To use this machine in production, IWC had to change the collet, which it did in its Cal. 8541B movement. Also, because the "Greiner" collet had not only a different shape but also a different size, IWC also changed the balance-staff as well.
Regards,
Michael
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