As explained a Certificate of Airworthiness was issued for three years. A condition was that an inspection was completed every 12 months known as an Annual. Also a simpler routine inspection every 6 months or 50 hours which ever came first. The difference of the Annual from the Certificate of Airworthiness was primarily: does the aircraft fly in accordance with the aircraft POH/Manual? This meant that if the aeroplane was approved for intentional spins then it was to be spun in accordance with its design criteria both left and right. It was also required to dive at the POH/manual VNE but not to exceed maximum RPM and also engine/propeller shudder free, aileron oscillation was a main concern to observe. The were many other checks with regard to turbo charging and constant speed propellers.
The performance was assessed with a 5 minute climb reducing the scheduled IAS speed by 1 knot per thousand feet.. This was to start from 2000ft AGL or higher to avoid turbulence. The height gain was recorded every 30 seconds and at each full minute the OAT was also noted. From this data you produced a graph indicating the altitude at each half minute. You then would find a mean altitude (not an average) and the mean temperature. From this you interpolated the mean ft per minute achieved at a mean temperature. This had to correspond to the POH/Manual. you were allowed, from memory, minus 30 ft per minute plus I think 50 ft per minute to the manual figures.
There's more to add but that was the basis.