At one point I was actually curious enough to buy and measure a model 69 barrel. The standard L frame one piece barrel uses, .562-36 threads and the 2 piece 44 mag is .638-36. Just .032 less than an N frame.

I believe he is probably correct using a 686 frame or an L frame 6 shot cylinder. I don't own an L frame 357 so I can't measure the cylinder wall thickness between chambers or outside wall.

But, here are a few things to ponder.

Colt Pythons have been made into 41 mags. Pythons have a 1.550 OD cylinder and L frames a 1.559

Plus, an L frame cylinder is .113 larger than an K, which would give you an increase of .0565 to the outside and the radius of a 41 case is just .0275 larger than a 357 case, so a 41 chamber on an L frame should be thicker than a 357 on an K frame ON THE OUTSIDE WALL, depending on the radius pattern of the chamber centers between a K and L frame. But. how much metal would remain between each chamber would be just as critical and the thickness between 2 side by side 357 chambers on a 686 would be reduced by .055. But then they make a 7 shot L frame 357. A 357 chamber is .379. that extra .379/6 is .0632. So in theory there should be enough metal

There is no reason in the world a 5 shot L frame 41 mag would not work built on a model 69 frame. Getting a 5 shot cylinder you could ream to 41 mag would be difficult

All interesting to think about, but as I am probably going to turn my lone 41 mag into a 45 colt because I have lost all interest in the 41 caliber, plus the fact I have no 6 shot L frame cylinders nor a 41 mag reamer, I doubt I will explore the possibilities farther that this discussion.