There is a slot on the bottom of the sill that receives the sill angle. If you refer to this page from our website, and scroll down to the second page, you'll see how they connect. https://mathewsbrothers.com/Portals/0/MathewsPDFs/WalcottPDFs/Walcott_CAD-Replc_Double-Hung.pdf
It's part of the exterior casing. It's screwed to the casing and casing is installed in factory.
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