I can’t answer your question as such, but the last Anson I was inside was C19, exTX183 and G-BSMF, at the Al Mahatta Museum in Sharjah. Its seats look like the “without headrest” you illustrate. Photo below:
I certainly hope it’s a typo. I last flew WZ860 with Oxford UAS in 1963, and it is now in the USA as N26JH disguised as a Canadian version with bubble canopy and revised undercarriage legs.
You say “especially” Wessex in you thread title, so are you looking for codes of other types? If so, I noted Wasp XT415 from HMS Danae in June 1968 carrying code 464, but no ship letters.