I found this subject while doing a search on the TDR. Being a collector of old clocks and watches, I thought I'd pitch in my 2 cents worth. In the days gone by, before electric, battery, and quartz became the norm, clocks where usually sold in either 30 hour or 8 day models. They could be either weight or spring driven, wall or shelf models. A buyer could look in a catalog and pick out the one he/she wanted. I have heard from other repairman that the eight day movement was designed to get a week on one winding with out it stopping. Makes sense to me.
Another interesting fact ( if anyone cares ) is a clock is only a clock if it strikes the hour. Otherwise it is a timepiece.
Chris