Think "Arma-steel" moniker was applied to cast cranks .. and rods?? that had been treated with a surface treatment to increase hardness, something like nitriding if not exactly that. It helps durability but does not increase core strength. They had to stop using that term as it was misleading I believe.
MH makes a distinction between case and core hardening, I believe the tensile strength numbers in the previous post were of core strength though I do not know for sure as I haven't read the whole "Standard Steels" section and am no expert. I believe core strength is what would matter with connecting rods.
There is undoubtably factors in design of the rod that make one better than another, but the raw material is pretty important. Think I would pick early SD 4140 rods over 5140, and late SD over them ... if I could find them
Wish SCAT would come out with a Pro-Comp 4340 SD I-beam with 7/16 cap screws. Maybe that's pretty much what a Molnar is?