Hi Neil,
I just had a quick look at that "logo-less" DR-7 on eBay, and although I'm of course by no means an expert on Sigma guitars - I just started to learn about them a bit more, also courtesy of the various contributors of this very thread - my "gut feeling" somehow makes me doubt that this is indeed an original Sigma...

For me the key indicators are the fact that it simply doesn't have a Sigma logo on its headstock plus its internal label doesn't mention that it's a Sigma either (and I've also never seen a Sigma yet that had a label that looks the same as this particular one), so I'm inclined to think that this is not a Sigma DR-7, but who knows (as the variations of this brand are seemingly alarmingly endless)...

I've now somehow become quite fascinated with those old Japanese-made Sigma's, so I decided yesterday to create a little database (in MS Excel format) for myself (and maybe some others will be interested as well one day?) & attempt to bring a little "method to the madness" here...:)

My "Sigma-database" will exclusively consist of Japanese-made models from the 70's (so the original Sigma line), and I've decided to add as many as possible data - as found on the www - to it (currently based on 23 different categories, e.g. model name, adjustable bridge or not, type of label, body material etc. etc.), and in this way I'll hopefully be able to get a better understanding of the many existing variations out there of this fascinating brand...

Stefan