I am building another close copy of the Bridger Hawken. It has rod pipes over 1/2" inside diameter. Here is one of them. Note the riveted rib.
The front end of the rib showing rivets and pipe.
This is a drawing of the Bridger Hawken made at GRRW in 1975 to which I have added my measurements. The pipes are 1 3/8" long, .615 Outside Diameter, .530 Inside Diameter and .060 thick. I had to make my own pipes by cutting them from 2" square steel tubing and pounding, grinding and filing them to shape. Took me 10 hours. They are the shorter ones on the right.
Then I went to Ace Hardware and found exactly what I needed in the bolts and screws bins. "Spacer" part no. 59370B for 95 cents, listed as 1.5" long, .656 OD and .500 ID. They actually measure .530 ID and are .055 thick. They are on the left. Ve gets too soon oldt und too late schmardt....