The Search service REST APIs and data controls are available only for search using Oracle SES. They are not supported with search using the original WebCenter search adapters.
It answered me. My requirement is without Oracle SES :/.
Why don't you try out this https://mgmt2.espressologic.com/Management/?accountType=eval#/ and get faster de-bugged RESTful service for your backend.
It will get you started within a minute and solve your http 501 and param issues in no time.
This issue was fixed a very long time ago regarding to Data Control that only works with SES.