Sorry, but that is pure BS. The steels or age of the gun have nothing to do with it. The old guns and the new guns are exactly the same in this regard - they were all regulated firing the right barrel first and still are. Some don't mind having the firing order reversed, and some do, regardless of age, end of story.

Yankin' the rear trigger first is just a crutch for those who haven't been able to master their double rifle yet. If you can't fire either barrel first without the slightest worry about a double discharge, then you just haven't mastered the type yet. It's true that some folks seem to have a really hard time with double triggers. All the more reason to learn it right the first time rather than create bad habits that will only get harder to break. Considering what large bore double rifles are intended for, it's really dumb to take a short cut through the learning curve.

Besides, the "rear trigger first" crutch isn't always effective with some rifles. Because the makers fired them in R/L order, sometimes gremlins appear when the firing order is reversed. At our last two Double Rifle Shooters Society events (the last just last month) I watched four double discharges with Merkel doubles when they were fired in L/R order. Last January, a .500 Merkel double discharged on a PH. Last month, a Merkel Model 141 in 9.3 double discharged three times in a row. I then fired two pair with it in R/L order without a hitch. Merkel isn't the only make I've heard of this problem with.