There was a time when being a telegrapher was an important profession. They provided local and world-wide communications via landline or wireless. As a professional, the telegrapher usually had his own key, because most railway stations and telegraph offices would have only provided a straight key bolted to the table. The telegrapher would take his "bug" to work as any craftsman would bring his tools. A delicately adjusted Vibroplex would not be just carried around in your lunch box, you had to have the official Vibroplex Carrying Case. These cases were always available from Vibroplex for around $5.00, but are now valued at $100 plus. The carrying case is leatherette (thin layer of leather) covered on the outside and felt lined on the inside with a heavy-duty leather carrying handle. The cases come in 2 sizes, i.e. the larger case for the larger Original, Lightning and Champion Vibroplex bug keys and the smaller size for the Vibroplex Blue Racer. The smaller size carrying cases are very hard to come by.
The latch on the carrying case keeps the bug key safe during transport, but the reason for the lock and key is a mystery since it obviously wouldn't protect against thieves taking both the "bug" and the case! Maybe it was just to deter the curious. The majority of Vibroplex carrying cases are missing the key, so to have a Vibroplex carrying case with lock c/w key is quite rare.
Measurements of felt lined key space: width 3.5 x height 3.5 x depth 8.5 inches.
The key is marked Nielsen Tool & Die Hartford.
ATTENTION:
To lock the spring-loaded latch, turn the key one complete turn to the left (anti-clockwise) and to open one complete turn to the right (clockwise).
In 1912 the company started as Nielsen & Wilson Hartford Connecticut USA.In 1937 with the passing of Wilson, Nielsen & Wilson became Nielsen Tool & Die Inc. Hartford Connecticut. From the key I estimate that the carrying case is the same vintage, i.e. late 1930s.

de Herman vk2ixv