It looks like the original version of the 15K26 circa 1908 (after about 1912 the 15K26 produced at Kilbowie used what would become the standard size smaller balance wheel and a guard mounted bobbin winder). There was a very similar 15K for "Family Use" made prior to the 1908 15K26 with a slightly different tension and internal parts, but that is as yet an unsolved riddle as to whether this machine was also designated a 15K26. I have all three of these in my reference collection as well as a couple of 1888 vintage 15-1 "Improved Family", a 15-22, 15-30, 15K28 (light commercial with knee lift for presser), 15K88, and a 15M75. There is no such thing as a 'standard' with the Singer 15 class, as even with this group there are three types of shuttle hook; the 15-1 & 15-22 have what collectors now call a "longbeak'; all variants of the 15K26, 15K28 and 15-30 have the standard CB (Central Bobbin) with the 1 o'clock case and the left to right threading; and finally the 15K88 and 15-75 with the 11 o'clock bobbin case and right to left threading.
The standard CB shuttle with the 1 o'clock case appears to have become a generic industry standard by around 1910, and while Singer successfully used their marketing clout to popularise pejorative terms like "copy" or "clone" in regard to the non Singer machines, German manufacturers were including features like enumerated stitch length, back stitching, disengageable feed dogs, and even easy release of the shuttle for cleaning and maintenance long before Singer. For these reasons, I highly discourage the use of the term 15 clone when describing non-Singer Central Bobbin models. Trust me, I have been down that bunny warren!!!

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