If you come across a Sigma missing it's logo and has the label removed or inside marking removed it's because it is a Sigma factory second. I recently purchased a Picador 12 string guitar for $100. After peeling the Picador metal label from the headstock I found the Sigma E and the faint outline of 'Sigma' still in the finish. The outline of the removed label is still visible thru the soundhole.
I am still trying to figure out the model number but it is a very early 70's model with an 0 fret, adjustable bridge and open tuners. I'll soon be turning it over to Roy Toepper at rtcusomguitars.com for upgrades and to clear up a bit of a clouded finish.
I also own a 52 SDR-7 that I just love. I still wonder though about the exact age of this guitar because this model was supposedly produced from 1979-81 yet the Head has the Sigma E and a paper label which supposedly ended in 77.