Over the past 2 days, I have performed this on a pair of Shields, and I believe that the dates are 1871 (first) and 1870 (second). Here are the pictures: The 1871 is my favorite of the 2, partially because it is a key date. I could likely get $10-30 out of the $2 investment. In my favor for the date is the mismatching of the 1's serifs. It's better than most of the 1868s I find when restoring! This one, that looks like an 1870, has a 50 cent investment. The date is barely visible, but it is there. This is why I clean the dateless ones up!
What would white vinegar do for it? I don't think it could help clean any metal buildup on the coin, which is likely what causes some coins to look so worn.
It’ll turn up a date on 75/25 CUNI good as anything, including Nic-A-Date. Those coins, they’re past-it, nothing will restore them.
I will admit that those, other than revealing a date, are truly too far gone to save something, but these are thankfully 2 of the lower grade coins I have restored. My best 2 have both been 1868s, one turning out in VG and one in VF.
If I an talking about the same thing you are, all it takes (surprisingly) is being put in water for a little while after soaking it. Then, I wipe it off with a paper towel. The 2 1868s were problem-free other than me Nic-a-dating them. I actually got $12 out of the better 1868 on Wednesday, and I have yet to sell the other one.