South Africa 1913 1/2d green (SG 3)
This is an odd one:

Note that while the top of the stamp is perforated normally, the bottom seems to have been perforated with a different kind of machine. I've seen this before on some German inflation-era stamps which are somewhat expensive on that account.
This one is also imperf on both sides. I don't think the side perfs have been cut off with scissors - you can see part of one hole and if there were any others we should be able to see at least part of some of them too. But this is the only hole we can see.
In short, an odd stamp!