This has been done to death
Not all parts are created equal, even if they are claimed to be "OEM". And I wish to highlight the previous quote marks...
I'm not going to go into it all again but suffice to say if you want genuine buy GENUINE from a dealer. If you must buy from an online store, make sure it's a genuine part that will come in original, unopened LR packaging.
And for the part you mention I would be getting the real deal...
Edit: Disco drop arm conversion a very good idea. Genuine of course