I shot my new to me 3913 yesterday. Today I cleaned three guns and the magazines. I have 6 S&W mags for the 3913. I took them apart, cleaned them and put them all back together. I do them one at a time to not confuse which parts came with each mag.

On about the 3rd or 4th mag, I noticed the butt plate catch assembly were slightly different. That particular one attached to the spring in both the front and back. I checked the earlier mags and noticed all the assemblies had a front catch. Only two of the six mags were attached front and back. The last mag butt plate catch assembly was plastic. It did not attach to the spring at all.

Do I need to attach the butt plate catch assemblies in the front? Shoud I replace the plastic part>

I had a 3913 .and I remember that S&W changed the way they put together the mags metal ....to plastic thingys and plastic base plates. I would mark that magazine and use it for practice ...see if it functions as intended. The other 5 should be good to go.

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S&W BG380, S&W 915 9mm

The latest variant of the single stack mags, all calibers, used a molded black plastic floorplate catch. It just sits against the spring and pops into the hole in the floorplate.
You could still get them for a while, but its drying up out there. The metal ones were the only thing I ever found to be rusty on 3rd gen mags.