There is a develompent in the Singer round bobbin models from model 15, 9W7 to 115 and 101 and 201. There's model 66 in 1900 and I have always though of 101 and 201 as a development from the drop in bobbin machine.
Some early German models are rotary, with similar bobbin case as model 15, but not identical setup. I don't know if it's possible to sort out the lines of development, dating and origins at this point, a 100 years down the road.