A while back we had a discussion on Sigma neckjoints, and those I had experienced by then were dovetails. However I have recently taken part in a discussion in a swedish forum, where a guy I know is working on a Sigma DM-5, imported and sold by Levin Guitars in the late 70:s.
I know Sigmas were sourced from several japanese manufacturers and some of them even had solid tops (however I have never seen proof of that). Some of those imported by Levin were rather indipendant from the Sigmas sold by Martin on the US market, except from model names, material specs and marking.
This particular DM-5 sample (DM-5 was a D-18 copy) had a cracked neck heel and were in need of a neck reset. The guy working on it couldn't get a grip on the type of joint and asked for advice. Now the neck is removed and I just thought I'd show the pictures. I have never seen this kind of joint before, and I can't say I like it.