bkay

I went out to check my Singers from that era, and most all have the groove down the length of wire. My mom's 1948 15-91 has the groove, my 1950 221 has the groove on one of the wires (both will have to be replaced), the 1948 201-2 that I gifted to DH's DIL has the groove, my 1951 201-2 has the groove. All the pieces of wired controllers I have also have the groove. The exception (other than the featherweight) is my Canadian made 15-91, which doesn't have the groove, but is similarly shaped and sized to the others. (Went back out to check, and the 301 I just purchased has the groove as does the black 301.)

I had to replace the controller wire on the 201-2 for the DIL's machine and the 18/2 wire from Home Depot fit perfectly and didn't appear different than the wire to the wall. I believe you are safe in using the grooved wire for authenticity and to fit the controller. Most of my machines are from the 1948 to 1952 era, so should be a good representation of what was used originally.

bkay