1, 6, 11 etc has of course the advantage of no smaller first interval. Following John's and Patrick's comments, the date sequence used on small C18 and C19 English clock date dials, when all dates are not numbered, was usually 5 10 15 20 25 31 (of course the dial had to allow 31 as the last as the hand or dial rotated in 31 days and had 31 divisions not varying with the length of the month). When the dial was a rotating disc showing through an open sector indicating against a pointer it went 3 6 9 12 ... 27 31, because closer numbers are needed to see where you are. To have 1 marked as such is unusual.
I think that it may have been felt that with the minutes being numbered 5, 10, 15 ... it would look odd to have the dates numbered 6, 11, 16 ... (as well as being something different to engrave or paint!) and then the 31 arose as a reminder that 1 was to be found next to it. Andrew James
