||The 261 is a variation on the most
important FC slide rule of that time, the 360. The difference is the
decimal figures 1.1, 1.2....etc between 1 and 3 on the upper scales. The
360 didn't have this decimal subdivision.
This specimen doesn't have a date
stamp, so dates from before 1920. It has a very special cross section
that is identical to the cross section of my 366 that dates from 1921.
So I would estimate that this slide rule was manufactured between 1915
and 1920. The slide rule does have metal reinforcement strips. This
patent forthese strips dates from 1907.
The box is of the type that is
(according to me) the most beautifull one Faber-Castell ever used.