The first PR9 ejection was from the first PR9.
That incident was the first "true looking"
PR.9, XH129, the previous
PR.9 was a wing-modified
PR.7 WH793.
XH129 crashed as you say (pilot was called Knight - JohnF will have the report about his person). Knight ejected but there was
no observer's ejection seat in that aircraft, it had a B(I)8 type nose (nose cone glazing). The observer was supposed to get out the same way B(I)8 navs did, through the entrance door.
That incident caused a series of mods by Short Bros which included an ejector seat for the nav. These mods were incorporated from the eighth airframe, XH136, onwards and retro-fitted to 130-135.