A bad pun is an authentically bad pun whatever its origin. No, I've no particular reason to suspect it of being from 1943 either, but then the lighthouse joke is always dated wrong too and nobody seems to mind.
And well to do so - in true urban legend style the same old stories are re-circulated year after year, always "new", "just released" under some obscure badly understood law or leak. These can give (to the trained eye) a veneer of horseshit to even a true story.

In this case the letter is dated 1943 - which means it could have been released in 1973 under the thirty year rule anyway. It has certainly been circulating on the internet for nearly 10 years and would probably have been circulating a lot earlier had the net been as popular back then.

The FOIA is useful for getting hold of *current* government operational data, and you would have to know *what* you wanted them to release to formulate the request. And FOIA only takes effect from this year. So there's no chance of it being relevant to this story. It is of course a weak attempt to make the story sound more convincing, which to any critical eye has entirely the opposite effect.

Now is the letter genuine or not? This column quotes it as genuine without comment, but as (now) a true piece of the folklore this says little. One route to an answer might be to check Russian court records for the period to determine whether the Turkish ambassador really was called Mustapha Kunt. The thread at <88d398$pnb$1@nntp8.atl.mindspring.net> (alt.folklore.urban) has more details. It appears the people involved were real enough and in the right place at the right time, however one would not expect Lord Inverchapel to misspell his own name (Clerk/Clark) on official correspondance (unless his secretary was a real klutz) and the letter appears to have been typed on equipment that did not exist until at least a year later.

A good, but imperfectly researched internal Foreign Office joke gone astray seems a likely option. Potentially a back-formation from alleged true events.

The really interesting thing is that that letter reads like Hunter S Thompson (on a calm sort of day, obviously).