I dunno about 309L. But... I do know that i worked in a couple of shipyards and have been around some form of metal fabrication most of my life and have never run across a need to weld dissimilar metals. Perhaps some pipe line special application or other industrial application. Not sure what drives the 309L sales in your area but day-to-day in a typical welding shop welding of dissimilar metals is rare.
Lil Red... you may very well be in good shape but I saw enough to be concerned. Having a career welder take a look at is a good move. Best of luck!