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I presume you mean the thing circled in red in this photo, which is not folded down. I have a '73 D-88 convertible.
I don't know for sure, but I'm guessing you'll have to find a whole new front seat. At least find a seat to take the armrest off of.
Does your car have one now? If yes, and it's in bad enough shape to need replacing, what's the rest of the seat like?
Certainly any '74 Delta 88 with the seat you want should be a donor car. I don't know if seats from the other full-size D-88s of that era ('71 to '76) would fit. I would think yes.
Thanks Jaunty75 and bigdooly for your suggestions. I’ll cast a wider net for similar era GM sedans as I suspect they’re mostly the same size. I wish I had the original armrest but it vanished a few owners ago. BigDooly you sold me the headrests a few years ago please let me know should you get your hands on a white armrest. And yes, it is the one circled in the photo.