This can be tricky. Especially as different companies have different ideas for what "OEM" means....
From your description i would hazard a guess that "normal" will be a cheaper pattern part. "premium" will be a genuine LR part, and "OEM" in plain box will be a part manufactured by the same company that supplies LR but in their own packaging, ie not in a LR box. But as ive discovered in the past, this is not gospel.
If its an "important" part ie something that is a show-stopper if it breaks, then buy a genuine part from your local dealer. Then your 100% sure its the real deal.
Avoid Britpart like the noro virus...... Yes some of their parts are acceptable but i wouldnt risk it. Hope thats of some help