I do not know about 7 day, but both 56 hour and 8 day chronometers were made in quantity during the entire life of the machines.
The reason was that there was a market for both. An 8 day was rarely needed aboard ship but some users wanted the safety margin of an 8 day. A 56 hour chronometer is wound every 24 hours and so is an 8 day.
But why a 7 Day , as you say they are wound everyday, why not a 4 (or 3 or 5) day and excercise more of that spring?
I can see a reason for an 8day; if you do wind it every 7 days if you forget on the 7th day you get an extra day's grace. But with a 7 day you just stop if you forget on the the 7 th day, very bad for an MC and its users!
I think you pose an imponderable. If there is a lot going on on a ship at war or in a storm, it might not be practical to wind it every day. There are some 30 hour chronometers, but the great bulk of them are 56 hour duration. The choice of 8 day duration may be related to the common existence of 8 day clocks. If you were asked to put in a lot of reserve, what would you pick?
My Hamilton model 21 is really a 48-hour chronometer that runs 52-hours on a winding. Much like I suspect an 7-day chronometer would probably run 8 days. I wonder if there really is any difference between the run times between a 7-day or an 8-day chronometer! I have two Seth Thomas World 15-day clocks that both run 21 days on a full wind.
That there might be such a thing as a 7-day or 8-day chronometer is a puzzle to me. When the vast majority run up to 52 hours on a wind. Might there not be isochronism problems with much longer run times? The only way I could see a 7 or 8 day chronometer not having isochronism problems would be a/ to wind it every 24 to 48 hours (kind of defeats the purpose of a 7 or 8 day run time), or b/ fit it with a remontoir.
In the recent reprint of Gould's book on the marine chronometer, page 217, he states the an eight day chronometer should be " wound daily, or at the utmost, twice per week." He also indicates elsewhere in the book that, generally speaking, 8-day chronometers underperform compared to the two day variety. So one might well ask, what purpose an eight day (or seven day) chronometer?
Thanks to you all for your replies and supplementary questions!
The maker of the MC made both 7 and 8 day MC's and the wind indicator runs out at 7 on the one in question. Mercer confirms he made 7 and 8 day versions too.
I came to it thinking it was an 8 day as it was described so, but the WI caught my attention and Mercer confirmed it.
The maker made over 1000 MC's and this one is right near the end and it was not a one off oddity.
An imponderable as Tom says, but teasing!