Spring
Well, I wonder why you think you need a new spring. My next question to myself was, are they the same spring for a pre model as they are for the newer models? You made me go to the safe and take a pre 27 apart to see if the spring is the same as for a non pre model 27. I only had one spring to compare it to, but it looks like my pre model 27 spring is longer and not quite wound as tightly. You might have to stretch a standard spring some, if it does not work as configured.