The 29-3E & 29-4 got two of the first "Endurance" enhancements that the 29/629's ultimately got. "The retention system on the yoke, or cylinder crane, was strengthened and studs within the frame were radiused to help remove metal stress."
The rest of the Endurance changes came with the 29-5 in 1990. "Now we began to see obvious outer changes in the Model 29 as the cylinder notches were made longer to prevent the bolt from jumping out of the notch upon recoil. At the same time the bolt was changed and the innards of the Model 29-5 were changed to provide a method of holding everything tightly together when the .44 was fired to prevent battering under recoil."
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