What was I using?

I’m pretty sure I was shooting a Pentax SP500 back then (in or around 1971). I don’t think I have any way of knowing for sure. I originally thought I was using Kodachrome at that time, as it was usually my film of choice. I certainly know it was slide film, because this picture is from slide film. Looking at my slides, though, I see that the results of my photography that day (or week, or whatever) were packaged in Mortifee cardboard. In other words, the slides didn’t come back to me in standard Kodachrome “slide cardboard”. That’s what makes me think the film might have been something other than Kodachrome.