Great question. It got me curious and I did a little digging, but came up empty.
The newest Butler tanker (liquid) I found searching Truck Paper was a 1978. So they might’ve been gone awhile and I never noticed.
Maybe call one of the “big” tank repair shops, like Jacob Press’ Sons. They might have some strapping charts on file. Just a thought. Good luck!
Please post how ya got one if ya get one. Thanks!