Yup, the original Koni shocks do that. My front left did exactly the same.
It started with a knocking noise, I couldn't find it.
On advice from a local camaro owner (not many of these in the UK) I removed the shocks and found exactly the same problem you have. The right side moved smoothly the full distance, the left side only about 1/4 inch then went stiff.
There is a design fault in the original 70s Konis that makes them fail this way. I got my rebuilt for not much money.
Other guys on the PBB in the US have done the same with Koni USA.
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