I wear a SZ 41.5 So Kate black-patent, can do a SZ41 So Kate black-suede..the difference is due to the flexibility of the suede vs patent (pretty stiff!)
I have the Ostri in a 42, it's a good fit for me. The toe-box is short-toe like the Pigalle 120 (pre-2013). I.e., the toe-box is NOT narrower than So-Kate (it;s wider), however the toe-box is *shorter*. There still may be a sizing issue, i.e. too small to accommodate your foot.
Based on your description, you & I differ by .5 size. So, you would need a Ostri SZ 41.5 (if I'm a SZ 42). Likely the SZ 40.5 you found is way too small!
Feet come in all shapes/sizes, so only way to know is to try them on. It sounds like it's used shoe..not able to return, or maybe?