OurWorkbench

Actually, the round bobbin was patented in 1878 per https://patentimages.storage.googlea...2/US208838.pdf And while I called it a long beak shuttle, the Singer Improved Family evidently called it ‘highpoint’ shuttle. The Improved Family machine was the precursor to the class 15 machines. You can read about the Improved Family machines at https://ismacs.net/singer_sewing_mac...g-machine.html and the highpoint shuttle can be seen about the middle of the page. I don't think there is any alpha as most (with very few exceptions) serial numbers I have seen have been pretty well centered on the plate. A picture of the bobbin area would help to confirm that it is probably an early 15. The 115 has a different bobbin area - there is a thread about the differences between a 15 and 115 at this thread Singer mdl 15 vs the 115, what's the difference? However, it is using a later class 15 as a comparison, rather than the early ones that used the IF ‘highpoint’ shuttle. Also if you look at this picture of the 115 https://www.quiltingboard.com/attach...806b6f793.jpeg you will note that there are three holes to the right of the needle area. Two side by side for attaching the seam guide to and one further back.

I'm still going with an very early 15k and it takes a different bobbin, which is fortunately still available at https://www.cutexsewingsupplies.com/...32221281812533
I would also like to see a picture of the hand wheel end of the machine.

From my understanding that the "C" serial numbers did not go as high as the numbers on yours.