Locksmithing during this era was considered more of a craft than a profession. It was often somewhat of a sideline as there were few locksmith-only shops. More often than not, locksmithing was offered in addition to bicycle or gun repair, or saw sharpening.
During this period, individual locksmiths usually developed their own tools and servicing methods. The craft was often handed down from father to son, keeping secret techniques in the family and away from the general public.
Locksmith distributors were few and far between. Most locksmiths had to purchase their locks and supplies from hardware stores, contract hardware and occasionally from the lock or key blank manufacturer.
Skeleton (bit) keys and flat steel mailbox keys were duplicated by hand with a file, a hack saw, Keil Turret key machine or Ilco Simplex key machine. The key machines would duplicate cylindrical keys and bit/skeleton keys. The Keil Turret machine could be used to mill the gates in the blade of a skeleton key.
During this time, there was little difference between commercial and residential locks. Rim knob sets were mounted onto the inside surface of the door. These locks required long, long keys. Some lock manufacturers made folding keys to get through the front of the door and into the lock. This made carrying the key in a pant pocket possible.
As doors became thicker, mortised locks took over. Mortise locks were available in bit, skeleton, and pin tumbler configurations. Pin tumbler (cylinder) mortise locks which had been available since the late 1800s, were used on front doors. Other bit key locks ranged from three to seven levers used on the back and interior doors. For privacy, skeleton key locks were also used on bathrooms, etc.
During the late 1930s, McKinney (today owned by SARGENT Lock Company) introduced a tubular lock with an adjustable backset. Also in the 1930s, Corbin introduced the first field-reversible without disassembly mortise lock.
From 1941-1945, lock manufacturers concentrated on the war effort with little time for new inventions. Locksmith techniques that worked in the 1930s did not have to be changed much for the 1940s. This is because car manufacturers stopped making new cars during the war years.
However, Curtis introduced their Post Type code cutter for originating automotive keys. It utilized a 'Post' for controlling depth of cut and special carriages which controlled the spacing Several posts and carriages were available, covering many of the automobile manufacturers.
In 1946, George Houlsey, Jr., received patents for a floor closer.
In 1948, Harry C. Miller invented a combination padlock to be used on locking bar cabinets. The following year, he invented and patented several manipulation-proof combination lock designs for safes and file cabinets.
A post-War housing boom spurred lock makers to develop new hardware which could be quickly and easily installed. Cylindrical designs first introduced by Schlage Lock in the 1920s became the lock of choice and changed locksmithing forever. Bit locks and rim locks lost favor as the public demanded a more modern appearance for their homes.
Many lock manufacturers produced their tubular/cylindrical locks with different sized cross bore, edge bore, and backsets. Backsets included 3-3/4", 5", 7" and 10", enough to bring the lock towards the middle of the door. This introduced the decorative escutcheon. At the same time, Russwin introduced the Ten Strike, a soon-to-be universal mortise lock that brought about standardization of the size of the mortise lock face. In addition, the internal parts were interchangeable, and could be converted to different functions.
In the early 1950s, Sargent introduced The Align-A-Lock, a 2-1/2" backset that would retrofit into either a 2-3/8" or 2-3/4" backset door preparation with a 2-1/8" cross bore.
In 1955, Lockmasters, a locksmith school for safe work, was created by Harry Miller and James Taylor.