I see this is an old thread, but it was still great to read so much information about the history of Sigma guitars. I have a DR-15 with a S/N of 35089 that I bought brand new in 1978 in Harrisonburg, VA. My DR-15 is pretty much the same as Bob Spradlin's except it does not have abalone inlay around the hole. Mine has alternating black and white inlay around the hole.   

I had contacted Martin a couple of years ago to see if I could get any information about my DR-15. The representative at Martin told me they didn't retain serial number records for Sigmas, but noted the DR-15 model wasn't produced by Sigma until the mid-1980s. It's good to know someone else purchased one in 1978.