Which means that they have indeed cheapened out with a recent refresh - which is a shame. I have the cable I describe on the headphones I am quite literally wearing on my head at this very moment (HD 600) and identical ones were present in 59x and 650 from the era. Glad to know, TIL.
Edit: the 3.5mm male to headphone is Sennheiser 81435 [0] the 6.3mm male to 3.5mm female that fits over it is Sennheiser 562507 [1].