I would hazard a guess that these relate to receiver (below) and tanker (above). Just a guess and I am sure someone will be able to clarify.
Easier to explain is Trial 306. Bomber Command Development Unit used to run trials. Some would be effectively in-house where a piece of equipment could be tested and operational procedures developed. Others were much greater in scope and requirements. One many of us took part in was Trial 505. This was flight carriage of the WE177 Instrumented round. Nothing dramatic for the crews involved we just had to fly it on typical sorties though there were occasional glitches but not no significance to the weapon itself.
Contacting the Air Historical Branch for BCDU documentation on Trial 306 might be worth while.