I saw an 18SX mast this week right next to a stock AL mast.
The old tang holes were full of pop rivets, and tang was attached 18 inches higher. I shipped them both to my friend in Vienna Austria years ago. I can get my friend to measure them- response by email I betcha!

Dacarls:
A-class USA 196, USA 21, H18, H16
"Nothing that's any good works by itself. You got to make the damn thing work"- Thomas Edison