About the only help I can offer is to point you to the manual: http://www.pentairpool.com/pdfs/39501-0028.C.pdf
If I were you, I'd go ahead an order a replacement weir for the U-3 -- but order from a long time pool company, like this one: http://www.a1poolparts.com/-strse-30...ABS/Detail.bok . . . and tell them it's for an OLD U-3.
I've never personally replaced a U-3 weir, and am not familiar with the attachment. So I have no idea whether there's an old version that differs from the current version. But a parts house will know.
Once you have the weir plate in hand, you should be able to get an idea how it attaches.
. . . Regardless, I'm pretty sure you'll need to carefully remove the excess grout.
Good luck!
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