Delays, delays. Okay, to your question above, the donor box is probably from either a 78 Monte Carlo or a 78 Cutlass, The brass inserts in that box that allow for the use of a tube with an invert flare fitting are pressed in or otherwise installed and do not want to come out easily. Left them as is-did not want to damage them jic.
This shows what I have versus what I would need if I was able to go with an O-ring elbow. The one on the left is what I was working with. This elbow and fitting have one bad issue; the fitting does not spin so the elbow cannot be clocked to get it into an optimum location. Did try to free it up by letting it soak in Deep Creep for about a week but no joy. I can introduce it to a vise and a flare nut wrench but risk tearing the tube so that is a non-starter. On the right is what would work, if I could lay my hands on one. Problem here is that this sample end is attached to a pressure side hose. By measurement, the nut can be either 5/8ths or 11/16ths hex and has 16 pitch threads. Under the hex head the amount of thread length is .404". From the shoulder to the end of the tube is .192". The pipe itself is 3/8ths tube. Dimensionally it has a long leg that is 3.075", and a short leg that is 2.00" To me the hex fitting reads like it is whatever thread would be found on a standard invert fitting.
This is a second comparison shot of the default elbow and fitting versus one that I found hiding in a box of carb parts. The found one is a hydraulic fitting that has an invert flare end and matching hex nut that has thread and pitch that matches the on the right. It will thread into the return port on the steering box, but without a matching shoulder in the port it will not tighten up.
And here is what I suspect the return line should probably look like. This is a factory line, provenance and application unknown, with a nut that turns and the double shoulders at the hose end. I have had to reshape it slightly due to it being abused at some point in its life but, in a test fit, it sat in position beside the pressure hose and looked fairly factory, (for values of that term). Again, the missing link is the shoulder in the port hole on the return side. This is probably the one that I would use as it seems to match the existing architecture the most completely.
565BB, when I did the research on the AC Delco fitting that was mentioned above, I found that physically it seemed to meet all the requirements I had, BUT.... whether P/N 19146526, or supercession number 36350210, it was just not available. I actually did order it under its factory number from Scoggin-Dickey and they took the order, accepted it, and then came back to me and said the part was not available and refunded my payment. Summit says they will take an order but that they will only be able to ship sometime in middle to late June.
Why do I think that, no matter who the supplier or brand name is, the source for this part is a manufacturer in CHINA and, due to Covid-19 and all the international trade restrictions that are in play right now, the bat eaters won't either make it or have them made but won't ship it because their panties are in a wad about something???
Needless to say, but gonna say it anyway, I am frustrated. The fast cure seems to be scoring a replacement brass shoulder insert for the return side of the steering box. They come as a kit and I do have one on order. As of right now it is somewhere between there and here, and when it will get here is a fat ??. It is SO BAD, that I spent four hours on Sunday doing shop cleaning just to get to the Goat Locker, the metal storage shed that holds all my POL inventory, just because there was a length of expanded mesh beside it that I will need to make a screen for dirt to get the rocks out!! The good news was that I found I had enough Prestone to fill the motor and rad. The bad news Is that I am down on the quarts of Royal Purple for my pending oil changes and there were no filters on the shelf. And I am still in rotation for work so getting to the Speed Shop is not easy. A measure of How Bad it is, is that I actually unboxed my new Miller 215, assembled it's stand, and set one on the other. Still have to do the assembly on the welder and get a bottle for it: more screwing around to get to the gas supplier and get a contract set up but this is all sidebar to my main problem.
Anyway, hope things are going better for you. Stay safe.
Nick