Tubing
This is the data I used on my 337 for replacement of that tube, I believe this is the one you are talking about. Hope it helps.
Tray
(1) Removed diagonal cross tube running from right rear wing attach point at Station +41 to the upper left longeron at Station +77. Replaced the cross tube with a ?Chair? assembly. Welded bracket to apex of ?Chair? for an attachment point for rear seat shoulder harness. Installed a ?Chair? type assembly between right rear wing attach point at Station +41, left rear wing attach point at Station +41, upper right longeron at Station +77, and upper left longeron at Station +77. At apex of chair assembly, installed an attach point to connect a shoulder harness for the rear seat.
(2) The ?Chair? assembly adds additional clearance to the rear of the aircraft.
(3) The ?Chair assembly was installed so as not to impact the freedom of movement of the control cable for the right flap.
(4) Supporting Data:
(a) Piper Aircraft Corporation Frame Assembly Fuselage Drawing Number 20875 with last revision 8/17/49 shows the diagonal cross tube being removed. It is listed as part number 76, is 1-1/8? OD, 0.035? wall thickness, and constructed of AISI 4130 Chromium-Molybdenum Steel Tubing.
(b) Piper Aircraft Corporation PA-14 has similar chair assembly in its original design, refer to Piper Aircraft Corporation Fuselage Assembly Superstructure Assembly drawing 11046 last revised 12/27/48.
(c) The ?Chair? support brace installed is constructed of 7/8? OD, 0.049? wall thickness, and AISI 4130 Chromium-Molybdenum Steel Tubing. A drawing and pictures are attached for further reference.
(d) The shoulder harness attach point for the rear seat is constructed of 1? wide, 1/8? thick AISI 4130 Chromium-Molybdenum Steel, with a 5/16? hole drilled in it. It is shown on the attached drawing and pictures for further reference.
(e) Station measurements are from Datum line located at the leading edge of the wings.
(f) All welding accomplished in accordance with AC 43.13-1B/2A Chapter 4, Section 5.
(5) Weight and Balance has been updated following this modification.
(6) This modification will be added to the list of items on the aircraft inspected in accordance with Federal Aviation Regulations Part 43, Appendix D ?Scope and Detail of Items (as Applicable to the Particular Aircraft) to be included in Annual and 100-hour inspections.