Hey, maybe that is it!
I am certain that (a) I would have had absolutely no idea what “chicken grit” was as a kid (I wasn’t a regular modelmaker–these kits appealed to me because of the horror aspect (I was into graphic gory things and those were pretty gory for the time period) and probably because they were cheap and I could get Mom to buy me a couple, rather than the model-making aspect, and (b) I might well have extrapolated “chicken grit” to “chicken fat” in my ten-year-old mind.
I can’t be certain, of course, but this does sound very plausible.