1951 Proof Five Shilling Strange Incuse Lettering

My guess:

Since the lettering is incuse on the coin, it must be in relief on the collar die. It seems as if some of the lettering in relief may have broken off the collar die, and some or all of the collar die was replaced.

What I do not know is how the collar die was made. If it was made in segments, which would allow easy withdrawal of the coin from the collar die, it follows that only part of the collar die would need to be replaced. This would result in the variation of the incuse lettering that you have pictured here.

My answer is a reasoned guess, beyond that, more information on the manufacture of this issue would be required.

beyond that I guess a letter, email or 'phone call to Llantrisant may help