Actually I have a very similar hounds fitting on my "new" mast (built earlier this year). The fitting is made by AHPC. It only has two holes and looks like a heavier guage of stainless but otherwise is the same.

For a while Mozzies were using the A-class style fitting that sits inside the mast with just the tang coming out through the mast. This looks very neat indeed, but a Taipan, in the states I believe, tore the stays shackle right through the fitting and when I asked for one AHPC told me they didn't recommend them for sloop rigged boats any more. So now they sell a fitting that looks just like the one you have.