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Engineer for BNSF here.
I only bought the assembled couplers once, on accident. I just clicked the wrong button on the order page. The only thing I did after opening the package was run a large flat file over the top and bottom of the shank a couple times to make the couplers operate smoothly in whatever cars they were put in.
Every other time I buy the unassembled couplers. I once packed a bulk pack in my grip to work on while I was at the away from home terminal hotel. I figured I'd lug this box of tools and baggies of the sorted parts around for a week or two until I got them done. Nope. First trip out, first few hours after taking a nap I had them all done! I had so many after that I was able to finish my fleet with extras left over for future purchases. They're all gone now and I just ordered some more.
People see the couplers on my models and realize what they are, then they notice they're on *everything* I have. I am always happy to point out the conversion wasn't as hard as I thought it would be, it wasn't as expensive as I thought it would be and it didn't take very much time at all.
Fort Worth, TX