So the brother-in-law turned up this evening with an old map:
…and wondered if I could date it for him.
After much googling I discovered this map:
…which is dated to 1746, a little older than my b-i-l thought it was. He was delighted to hear that it was drawn over a quarter of a millennium ago.
But I was not completely satisfied with that date, and eventually tracked down the real original; the date was not 1746 but
I am kicking myself for making such a horrendous mistake; wrong by several decades.