Nigel at campyolody.co.uk has them:
"The "signet ring" toothed insert that fits between upper body & dropout on Victory, Triomphe and early Athena rear mechs #7116067 £18.00"
but he is out of action at the moment. Search on that part # though. If it is Athena, it might not be worth it; you might be able to pick up one in poor condition on ebay... Failing that, just fashion one yourself out of some suitable material; it isn't doing much structurally.
Nigel has the hanger bolts too if you are also missing that.
Nick