It will look just fine. I went back to double check and looked at everything to make sure I had not made a mistake. The oldest one I have now is my Mother's 15-91, which is pictured below. It was allocated in Nov. 1948. That one, I am sure has not been re-wired (although it needs it). All of my machines from that 15-91 to my 600 has the wire with the groove in it. That includes 2 - 503s, 2 - 401s, 2 301's and the ones that were mentioned previously.
(I attempted to photograph the 2 kinds of wire I have, but I couldn't get good resolution.)
bkay
Here's the 15-91 that belonged to my Mom.
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