Two-letter abbreviations with slashes

In English, most of our two-letter abbreviations are written as either two letters by themselves, or two letters with one or two periods.

But there are a few that are written with a slash between the two letters.

If anyone knows why that is, I'd be interested to find out more; TSOR hasn't turned anything up.

But mostly I'm writing this 'cause I think it's an interesting phenomenon, and I wanted to put together a partial list.

c/o
care of. This is the one I'm most familiar with and see (and use) most often.
l/c
lowercase. As used by editors to suggest making a word lowercase. Also u/c for “uppercase,” though I think I see that more rarely.
N/A
not applicable. Almost always written in uppercase, unlike a lot of these. Another very common one.
w/o
without. I'm particularly intrigued by the two-letter slash abbreviations for single words. Note that w/ is often used for “with,” so I guess w/o for “without” is a natural extension of that.
b/c
because. I see this fairly often, but it's always seemed weird to me. Most two-letter abbreviations are short for a two-word phrase; even “without” could be thought of as “with” and “out.” But why would an abbreviation for “because” include the C? I suppose you could break up the word into “be” and “cause.”
w/c
week commencing. I had never seen this before British members of the SH staff used it; I was initially sure it must be a typo, perhaps for w/e, which I think I've occasionally seen as short for “week ending.” I'm guessing w/c must be more common in the UK than in the US.

Wikipedia's discussion includes such abbreviations as r/w (“read/write”) and i/o (“input/output”)—both of which I think I usually see in uppercase—but those seem to me to be in a different category, because the phrase they're abbreviating also contains a slash. In the abbreviations I'm talking about in this entry, it's not clear to me why the slash is there.

Wikipedia also lists b/w; I've certainly seen that meaning “black/white,” which goes in the same category as r/w, but Wikipedia says it's also used for “between,” which I don't think I've seen before. Urbandictionary backs that up (and notes that it's used in text messaging); but then again, the first search result for [b/w between] is a forum discussion in which everyone but the original poster says that b/w for “between” would be confusing.

Which of course is a good reminder that there are presumably zillions of abbreviations that are used in some groups and subcultures without being known to society in general; hard to say where to draw the line. So I don't intend my list to be canonical or complete; just a sampling of some common ones I personally have encountered.

Any other particularly common two-letter abbreviations with slashes?