Proto.....Here is a paste from way up this thread from Stirling Webb,a well advised contributor. He has not responded in a long while so I fear he is no longer with us. AS FOLLOWS............... "Also made in small numbers in 1979-81 were Sigma models with the prefix 52S, as in 52SDR-7. Curiously, they are made entirely from some very strange laminates. The 52S series were sold as "professional" instruments; they have a pure clean resonant tone (like a high-end maple Gibson) which combines well with the human voice and records cleanly without a fuss. (I've had three of these, and all had thoroughly rigid construction, dead straight necks that had not moved in 25 years, low fast action, and great playing ease, as well as fine sound.)

Cheers   Neil