Hw70
As you thought, this pistol was introduced in 1970. The first models up to SN 66000 were considered unsafe, as the chamber cap could fly off and hit you in the face! The gun was modified after this SN in 1976, so be careful what you buy if you intend to use it. The optional safety catch was introduced in the late 1990s and the black version in around 2000. This data is from John Griffiths's Encyclopedia, but I can't find any info when the brown stock design changed. I had one of the first models and always found it to be disappointing. It wasn't really a target pistol, and didn't have enough power to give tin cans a good clatter.
Life is to be enjoyed, not endured.