Wolff Gunsprings sells two series of recoil springs for the 3-1/2" pistols. The ones they list for the Colt Officers ACP with the original guide rod are a single spring with the guide rod end wound tighter, to fit the .250" diameter guide rod. Wolff also has a spring for the Para-Ordnance P12.45 that's the same spring, but with the guide rod end wound to fit a standard .330" guide rod.
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