As always, you chaps have been a great help!

Thank you all for taking the time to write thoroughly detailed replies.
Thank you too Downunder, and Buchanan, for the photos, and for the specific measurements (Downunder)
Though I really want to make a brass cantle, this one will be a bit of a hybrid.....
and look a bit old.
Will show a pic or so in the DIY when it's done.
The old catch variations fascinate me, and I'd love to have a bash at some of them!!

Thank you all again!
R.