OK, BNP. British Nitro Proof, so it was definitely in Britain.
I THINK the SP stands for Special Proof... Hum... maybe not. Something I just found on line says that SP was used until 1925 and was a voluntary black powder proof. I don't know why it would have been used on a gun made well after WW II, unless it's because the .38 Spl. was once available with black powder loads...
The other ones I'm not familiar with.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.