Incidentally, and at the risk of a bit of thread drift, I actually witnessed the first flight of the Twin Pioneer (c/n 501 G-ANTP). Our kitchen window overlooked the Scottish Aviation empire and a first flight had been anticipated for several days. My memory is that it took place early on a sunny morning (when not too many people were around) and it was wonderful to witness the great event. Google tells me that the date was 25/06/55 - 60 years ago and I was at the time a 14 year-old founder member of the Prestwick Spotters Club.
In these days it is possible to register your aircraft with any registration that you like provided that the registration has not been issued before. In those days the Air Registration Board would not countenance out-of-sequence registrations and Scottish Aviation were considered to be exceedingly clever in managing to register G-ANTP when they did, so that they obtained "TP" as Twin Pioneer.