Just to put a bit more information forward regarding the Sigma DM 4 model I'd flagged up a couple of months ago ...
The guitar had originally been purchased by the seller as a brand new instrument in 1983, from a shop in Liverpool.
The DM 4 model is based on the Martin D 18.
It was made in Korea for C.F. Martin & Co. - this is branded into an NFL football shape on the internal back bracing along with its serial number and model number.
There is no paper label inside the body.
The 'propeller' inlay at the 12th fret position is alternatively referred to as 'cats eyes' - we'd wondered whether this inlay design was significant to date of manufacture or materials of construction?
Any assistance would be greatly appreciated.