Singer New Family 12 or 12k and Singer Improved Family 15.
Both of these machines are known as Fiddle base, as they have a curved base, unlike the later machines that have a rectangular base. The images below are of the machines as I found them “in the wild”. Slowly I am cleaning them and preserving their decals. Neither is perfect to look at, they both came in their original treadle tables. They both sew a straight stitch.
These are my oldest machines and more details can be found at ISMACS page The Singer Improved family.
2078448 377648 is 1875 , New Family 12
8193651 is 1888, Improved Family IF-15 (unlike most Singer machines, this serial number is found on the back of the machine.)
In May 2023 I found another Improved family IF-15, I added it and its table to my collection, her decals are even nicer than my first machine and the table is in better condition also. I donated the first machine 8193651 to Annie Bonny’s Locker Museum in Hawthorn, Vic.
8265113 is also 1888.
Side by side
The 12 series is much smaller than the IF 15.