Thanks for the reply
Looking at some sites like this one that have OEM parts, the springs are listed as compatible for the 535/528 and GT cars but not 550i:
https://www.getbmwparts.com/oem-part...IFJlc3VsdHM%3D
Definitely a PIA to figure out what works and what doesn't, but I guess since this site says that the spring will work with both I will get it here
UPDATE: Digging deeper, it seems like the ones I originally found are OEM. The part number is the same across multiple manufacturers and as per the link above and other websites/guides, it is compatible with the 535s/528s: 33536794653
Bilstein, Kilen, Lesjfors, Roc and Triscan all manufactured various springs for BMW over many years but they are all the same "OEM part". This digging is probably what made the seller slap 528i on the listing, they didn't want to figure it out
Might be helpful for someone that stumbles on this post later on