SIG Sauer is now making polymer magazines for the SIG 751 which are patterned after the SIG 550 magazines. The SIG 751 SAPR is essentially a scaled up 7.62mm version of the SIG 550.
UPDATE: SIG has told me that the SAPR has never been in production. So it is more correct to say that the latest SAPR prototypes have polymer magazines (the older prototypes had metal mags).
[ Many thanks to Aurelien for proving the photo and information. ]Related