I had a pair of Leitz Trinovid 10x25s c.1982 and the ring that held the cord was on an eyecup, which broke. Hard plastic on these and your photo looks to depict an aluminum part?
I made a length of cord with hangman's knots at both ends which were placed around both slide out eyecups and cinched. This worked perfectly.
I used them like this for a few years and, curious, then contacted Leica to see if I could get a replacement part. I was told the part was no longer available but that they would give me a credit for the full retail amount for the Leica versions as per 2001 when this dialog occurred.
This amounted to $500 or so of which I applied to a pair of Leica 8x32 BNs and I think my balance was about $475. I originally paid $225. for the Leitz.