Thick hardwired line cord manufacturing date range?

[xylenol15]

Hi all, I just lost this auction (I did make the other bidder pay for it though  )

Anyway, I and probably others were drawn to it by its shininess (which is odd coupled with that stretched out cord, maybe the last owner was a phone collector?) but also by the thick, heavy line cord with the 4 prong plug. Most of the hardwired phones I've seen have a thin line cord which I personally dislike, partly because it looks and feels flimsy to me and also seems to have a sticky quality that attracts dirt. It looks like they switched to these heavier cords later but only produced them briefly until the advent of the RJ11 plug. I think these heavier cords also had a sleeve near the end, which I think looks a lot better than just having that oversized hole near the end of the cord. Does anyone have a date range for their manufacture?

[xylenol15]

Ah, there, found a picture of the grommet I was talking about.