I ended up using what I had to make it work, but I would be interested to see what you have. Can you post a photo of the end of the stringer with a tape measure for scale and then an overall length that they are.
Difficult question. Stringers are 4 ft. long. Imagine a guy on his knees on top of all 4 boards in place facing the main tube. He has the stringer in left and right hands and rotates (using his wrists) the entire stringer in a counter clockwise motion about a quarter turn thus locking boards 2 and 3 and portions of boards 1 and 4. To keep stringer from sliding, boards 1 and 4 have stop blocks. The opening in the stringer is 3/4" at the top but at the bottom of the opening it is 1/2". (Imagine not a perfect U...again 3/4" at entry and bottoms out at 1/2")