Not 100% sure but I think the FS2 uses the hubs from the RB5SP2 / UMW517 conversion. If that is the case it uses the revised hole positions which are effectively half a hole spacing further outboard (i.e. the extra holes on the ali hubs) so that is the difference.
As for the wishbones 521 comes with an RB5 front wishbone on the tree and the LA272 comes with a ZX5FS/SP front wishbone on the tree.
As I say I am not 100% sure because I don't run either the RB5-SP2 or ZX5-FS2 as they both arrived too late for me to change to this season (I have a rule that the chassis I am running at the point I enter the season is what I use all season as I get no time to test mid season ... no matter what improvements come out)
Nick Caro will know for sure, he runs both SP2 and FS2.
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