Look what I found! A 1934 movie on the production of Singer sewing machines and all that it entails.
http://ssa.nls.uk/film/1592 - Its fascinating! and it shows amongst others the production of the Singer 15 in Kilbowie, Schotland. Making that the 15k88 and treadle. I enjoyed the lengthy movie (71 minutes).
Giving credit to where it's due: I found it after reading this information
http://www.singersewinginfo.co.uk/kilbowie/ on the Kilbowie factory. They mention a link to 'Birth of a Singer Sewing Machine' and I googled it and found...
Hope you enjoy this too!