A couple comments.
Rio Grande is actually VFF not VFN. My original source was Chuck Wyatt... perhaps in 2000 or 2001. I know Fusion is saying that my pic doesn't look like a large plum, but mine were always variably shaped, never always the same on 1 plant. It's often described as "blocky round" and sometimes mine look more globe or deep globe shaped than plum. The plant has dark green rugose leaves.
I looked at TGS as well as Tanager's and both have different looking shapes. I think I may even have a seed packet somewhere from Pennington Seed and they also look more deep globe than plum if I remember correctly.
As for the white line you're talking about, I've never noticed it, neither with Chuck's or Pennington's. Maybe you can post a pic of it.
Another pic of Rio Grande:
http://www.tanagersongfarm.com/heirl...ogrande00w.jpg