ern Pennsylvania. The keyway
marked “ 50” was the master. The
smaller lock, from OSU, has the
keyways marked “master” and
“change”. These smaller locks
were probably used on lockers.
Yale many times would use a
restricted keyway for the master
Yale also used the Bicentric
concept in key-in-knob locksets
(figure 7). I have not found any
catalog information on these
locks. This one appears to be
from the 1970’s.
Yale also had a much earlier
key-in-knob probably from the
1920’s. The two cylinders were
part of the knob, figure 8.
Yale was not the only company
to use two separate plugs, on the
master key and the other for the
operating key. Corbin also had
at least one model. There are
only two of these locks known,
the one in my collection and the
one formally in the John Giusti
collection. They must not have
made many. The padlock appears
to date to the late 19th century
US Expanded Lever
Yale did not just limit the use
of two keyways to pin tumbler
locks, they also used the concept
on a lever lock known as the
“DUPLEX” (figures 10 & 11).
(Note: Figures 10 and 11 have
the master and change keyholes
reversed, it doesn’t matter which
side is used.)
Common master keying in
lever locks uses one keyway and
levers with several gates. This
compromises security and limits
key changes like the single keyway pin tumbler cylinder does.
The Duplex has a set of levers
with just one gate in each lever,
the height of the levers control
the gate combinations. The master will have all the same heights
and the change will have varying
heights. Note that the master key
and operating keys have opposite
bushing wards so that the keys
cannot be interchanged.
Lever two keyway master
keying did not seem to be used
as much as pin tumbler. I have
only two examples of US companies using the Duplex concept.
Because both are extremely rare
they probably were not used for
very long or in great numbers.
D.K. Miller Lock Co. also used
the Duplex lever with two different size key holes for the master
and change keys. For years I noticed that the original blueprints
for the D.K. Miller 6 Lever Push
Key locks showed two different
thicknesses for the change and
Note: in figure 12, that the
Master key specification is 0.041”
and the operating key is 0.050”
thick. This mystery has been
solved. But where are more of
these locks? Please contact me if
any of you have another Miller 6
lever Duplex-type lock.
When I keyed the lock in
figure 13 and measured the size of
the keyways I realized why there
are two thicknesses. The operating key is too large to enter the
master key opening.
Continued from page 26
6. Two Sizes of Bicentric
7. Yale Bicentric Key in Knob
8. Cast Knob with 2 Cylinders.
9. Two Cylinder Corbin Padlock.
10. A DUPLEX Yale Padlock with
Flat Shackle and Original Keys.
11. A DUPLEX Yale Padlock with
round Shackle and Original Keys.
12. Actual Miller Drawing of