All 1970 halves are 40%. What you have their is the proof version. They also made 1970D that was the uncirculated version. All 'S' mint mark halves are proofs
What Magma said, all 1970 halves are 40%, none were released for circulation, the '70-D was only in mint sets and is generally considered the more desirable coin, the '70-S was only in proof sets. I'm also proud to say that I'm also an owner of a wild-caught '70-S.