Reynolds 531 is tubing that was introduced in 1935 and still used occasionally even today for traditional bicycle frames. Reynolds 753 was introduced in 1980. It is essentially heat tempered 531 which allows for thinner walls. A lighter bike results, but it is more expensive and difficult to work with. So maybe there is a 753 McLean out there - certainly an ultimate bike. Dale Brown is the gate keeper for all things McLean. Following is part of a response to the posting of the Performance catalog.
I had forgotten (or not sure I even knew) that McLean agreed to be in the Performance catalog.
Also interesting is your mention of Reynolds 753. I remember him sending back the qualification kit. He shouldn't have had any problems there as he only used high (56%) silver brazing compound but I am not sure if he ever actually made a 753 frame! Again, I never saw one nor heard him talk about making one for anyone... But who knows? Be nice to see it if he made one.