The [Concise New Partridge Dictionary of Slang and Unconventional English][1] (google books) gives the following sense of **bark**:

> the skin  noun  1758  UK

My sense of hard bark on someone would be akin to thick or tough skinned. I think the usage of bark=skin pre-dates the cinematic era as posed in you question, but I too cannot find any reference to **hard bark** that predate the cinematic era other than those you posted.

  [1]: https://books.google.com/books?id=GIuEAgAAQBAJ&lpg=PA34&ots=3o3LW-Qlyx&dq=bark%20noun%20partridge&pg=PA34#v=onepage&q=bark%20noun%20partridge&f=false
  [2]: https://www.oed.com/view/Entry/15567?rskey=tuLk3w&result=1&isAdvanced=false#eid