This should be the base-api ID. I have attached a snippet of my response below
HTTP/1.1 200 OK
Content-Type: application/json
X-Tyk-Base-Api-Id: d75faff558854777a6de08171be1121a
Date: Tue, 15 Aug 2023 10:14:42 GMT
Content-Length: 362
Connection: close
{{ Body goes here }}
Can you try using the versions API endpoint to confirm the base api is indeed versioned