Hi, I saw saw the pix on Flickr and got really excited! I own an identical SG special with the serial 105505 which must have been made the same week if not the same day as yours! I bought mine secondhand in July 1975 (still have the receipt) as a 19 year old in my native Yorkshire and the guitar is still completely original. (let me know if you want to see ant photos) The guitar was not very old at the time and I always thought it was a 73 model because of the schaller bridge. However if you look at the 73 catalogue they had dot inlays. Block inlays came in the 76 catalogue but had a smaller tunamatic bridge on the SG special. Besides I have the 1975 receipt. My best guess is its a late 73/74.
Whats the history on yours?