Does anyone know if there is a "standard" for labeling 2x2 cardboard coin holders in terms of what information goes on the holder and where?
As others have said, there's no standard, you're free to do whatever you like. You don't even have to do all of your 2x2s the same way, though if you're a dealer or seller, it's good for your customers if all the information is in the same place on all your 2x2s, so they're not wasting time hunting for the information they need.
My personal system is similar to what others have said.
On the front:
- Top left: country. Below that, denomination.
- Top right: date, parallel with denomination. Mintmark (if present) directly below the date.
- Below the coin: Any special notes, varieties or other comments about the coin.
- Far bottom right: price paid code.
On the back:
- Top left: catalogue number. Krause for modern, Sear for ancient, Spink for English hammered, etc.
- Top right: Sap number. Each coin in my collection has a unique number, which among other things helps me find it on my computer, both in my database and in my picture archive.
- Bottom: provenance details: usually just an abbreviation of where I bought it and when, though if further provenance is known I record it here too. I also record a "case date", when the coin went into that particular 2x2 (if it wasn't the same date as when I bought it), so in the event that I've bought a bad batch of 2x2s I can track down it and the other coins that were encased at the same time.
I don't record the grade on the 2x2, mainly because I realize that my grading standards change with time. I've been putting coins into 2x2s since I was ten, and the grades I put on those earliest 2x2s are laughably naive to me today. I sometimes also re-use the same 2x2 if I buy an upgrade.
Also, what type of pen/sharpie is safe to use?
Again, I don't think it's really a major concern, since the ink's on the outside and the coin's on the inside, and the amount of ink we're talking about is usually quite small. My choice of pens is dictated more by ones that I'm forced to write neatly with, since my handwriting needs all the help it can get.
But if you're worried, try this simple test: colour in a square on a piece of paper, and leave it on a table or benchtop to dry overnight. Come back next morning and smell it. If it still has a strong inky or chemical smell, then it's the kind of ink that could potentially cause problems if a lot of it is sealed up in a small space with your coins.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis