The slide plate (to access the bobbin) had a small oblong hole which aided whilst sliding the slide plate to the left to view the bobbin.
The Singer 15K was manufactured over many years from 1910 - 1964, then reproduced again in 1982 called the Singer 15N.
Unfortunately not many parts are available for machines of this age.
This one is a vintage Singer 12K sewing machine and was manufactured at the Singer plant in Glasgow, Scotland.
The Singer 99k sewing machine is small in size measuring only 12-1/2" wide, this image shows an early version the 99K (99K21 - 26) made between 1924 to 1954. The stitch length was adjusted via a screw and the machine did not stitch backwards.
Later versions 99K31 had a stitch length up-down lever with reverse stitch and numbered tension dial - Year 1955 The Singer 15K is always recognised by its tension unit being on the left face of the machine.