Now that I've taken mine out a few times, some additional thoughts:
The B&T rail panels for the hand guard are no longer made. However, since rails are standard, you can by Magpul MOE panels of the correct size and they bolt right onto the sides (they clear the barrel inside too).
The "old" version of the 553 with diopters does not have ambi sling hooks. The rail top version has sling hooks for a lefty (on the right side). You can get universal sling loops at stores like Blue Force and can run a single point sling.
Shooting with the B&T rail mount and a red dot, the Sig Romeo 5 high mount allows you to still use the diopters but the site is too high for a cheek weld. Personal preference but aside from the picture here I run mine with just the diopters and no red dot.
The clear mags pictured I purchased from a vendor that is selling out the last of their inventory. They are labeled "Dominion Defense" and appear to be made of the same material as original Sig. I bought them to use as shooters. Mags will be tough to get.
The weapon is solid! Best feeling AK-type weapon I have fired. I'm not crazy about the two stage trigger (I prefer single stage). Something like an HK roller lock rifle has to me a still better recoil impulse but this is still a joy to shoot. I trade between this and an AR type MK18 for my favorites. As a lefty I found the HK53 charging handle difficult to manipulate but my favorite of all rifles goes between the MP5 pattern and MK18 (like an SR-15 CQB).
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