Is this part for an Elna Supermatic or Singer (ca. 1950's)
 

https://photos.app.goo.gl/9ve3RmZFGXqMq5WS6Amongst my grandmother's sewing goodies, I found this mysterious cylindrical solid steel rod and cannot figure out what it's for. She had an Elna Supermatic (ca. 1957) and multiple same era Singer's.
About this piece:
Approx 3" long
Approx 1/2" diameter
Solid metal piece
A single etched line about half way down
Etched marking "660-Y"

To be honest, I'm not certain it's even a sewing machine part, but I feel like if it is, it's related to thread -- like a thread guide of some sort.

Any help or guidance would be greatly appreciated! https://photos.app.goo.gl/9ve3RmZFGXqMq5WS6

​​​​https://photos.app.goo.gl/9ve3RmZFGXqMq5WS6

Thank you, kindly!

I am new to this forum and not certain if the image posted...https://photos.app.goo.gl/9ve3RmZFGXqMq5WS6

https://photos.app.goo.gl/9ve3RmZFGXqMq5WS6

Welcome to the QuiltingBoard. It's not like any part I've ever seen on a Singer. Based on the dimensions you've given there's room for a Singer part number, which if it were a Singer part there would be. I can't say if it's Elna part or not.

Could this be an old capacitor? Perhaps it was removed from a sewing machine motor, but saved "in case?" I've only run across one Singer motor with a capacitor (wired alongside the motor), but some had them.

I was about to suggest that it looks like a capacitor. Singer used capacitors in some of their button style foot pedals. I have an idea that they may have used them internally in Featherweights but I'm unclear about that. Elna also used them in foot pedals and if I'm not mistaken in the machine itself near the electrical connections.

It looks like it may have connecting points on the left end where it might have been soldered.

See here: https://www.youtube.com/watch?v=iMOwDH8yLUM

It resembles the capacitor used in Supermatics but the Supermatic ones I have seen have a diagram of the circuit etched into the casing, and lettering in multiple languages. Perhaps this is a replacement capacitor manufactured by a third party.

Supermatic capacitor for comparison purposes.