Musgrave .38 Special

Yesterday, I was sorting through several buckets full of .38 Special fired brass, mainly range pickups. Most were fairly ordinary, but a few were a little out of the ordinary (including some very old ones). Among those were some headstamped: “mus K7 38 SPL” It looks like “mus” is Musgrave (South Africa). Were these widely imported into the US? I ask only because I do not remember seeing any like that previously. Also, what’s the meaning of “K7”? (K and 7 are on opposite sides of the primer)