NavList:

Thank you to those who have contributed to this discussion so far.  As far as I can see, for a 40 year calendar, we’re looking at a total of 21 year sectors.  19 sectors contain two years, and 2 sectors contain only one year.  This means we cover 40 years.

We also have seven month sectors.  The months appear variously displaced.  Ten months appear once, but January and February appear twice, once in black and once in red to indicate leap years.

Coincident and locked with the year sectors, we have 21 day of the sectors covering a period from Sunday to Saturday three times.

Coincident and locked with the month sectors,  we have columns of dates from 1 to 31 in weekly calendar form. 

AdrianF has indicated that we could turn this ‘circular slide rule’ into a linear ‘slide rule’ by unrolling it.  The next step is to work out how the years and days of the week are locked as they are, and how the months and dates are locked as they are to get the correct results.

What we no longer have is the wrapping paper to presents arrived in.  It might not have been possible for the manufacturer to solve the markings on these discs perfectly.  Therefore, there might have been a set of instructions such as “For years on the lower row, move the disc one space anticlockwise”  DaveP

As I was going to St Ives,
Upon the road I met seven wives;
Every wife had seven sacks,
Every sack had seven cats,
Every cat had seven kits:
Kits, cats, sacks, and wives,
How many were going to St Ives?

Weekly Magazine of August 4, 1779