I don't know if you can weld onto those plates and not pop any of the concrete behind them. If you can, you could weld large pieces of square tubing on them, (10"-12" long), using the tubing as pockets for the upright timber posts. (sizing the tubing according to the size of upright posts you will be using.) Drill thru the tubing and thru the posts, bolting them together. Then build the railings from there.

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).