Odd sized compression nut

I recently replaced a faucet in a medical facility. The SS flex lines that were suppling the faucet are not made anymore. The angle stops appeared to be 1/2” compression but when I tried a 1/2” compression x 1/2 Fip supply it would only screw on the valve a couple of turns and would not seal. I tried a 1/2 compression nut and the same thing happened. It seems like the the thread count is different or maybe metric? The valve is American Standard. Hard to replace the valve because there are no isolation valves on the riser. Anyone ever experience this or have suggestions?

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