“Mercutron” is a brand name of Mechanical Enterprises for their mercury-contact switches.
Few details about these switches are known. It seems that at least four series of Mercutron switches were produced.
All these designs work on the principle of a flexible mercury-filled tube, that is pinched shut by the switch to separate the mercury. One advantage of this is fast, bounce-free switching, as well as long switch lifetime. As the contacts are sealed, the switches are protected against environmental influences.
US patent 3600537
US patent 3600537 (filed in 1969) is the oldest Mechanical Enterprises Mercutron patent, for a conductive-liquid snap-action switch. It seems that this was not intended for computer keyboards, as it is shaped like a microswitch.
Behavior Research Methods & Instruments, 1969, Vol. 1 (8) page 329 (PDF page 1) gives a description of these switches under the heading “Subminiature Switches”. They are cited as having the “mechanical advantages of a snap-action switch with the electrical properties of a mercury switch.” They are said to have no bounce, and can switch “60 na” at 24 V DC. (Surely they meant milliamps?!)
US patent 3707611
US patent 3707611 (filed in 1970) depicts a keyboard switch with an external return spring. Wireless World, May 1972 page 251 (PDF page 49) contains an advertisement for “Mercutronic” switches from Tekdata in the UK, with a illustration of a switch similar to that of US patent 3707611, but much lower profile (the base area is drastically shorter). Based on a different advertisement from Tekdata (Electronics Today International, November 1974, page 7), it is clear that these are indeed supplied by Mechanical Enterprises. These switches are rated at 25 million operations, and are noted as being impervious dust, noise and electrostatic charges. (The unspecified “Mercutron” type in the second advertisement give a 50 million operation lifetime.)
Modern Data, April 1970) includes Mechanical Enterprises in list of attendees of the 1970 Spring Joint Computer Conference, noting “ME’s Mercutronic Division will display a series of interchangeable-key keyboards to be custom-built for the OEM user.”
US patent 3845264
An advertisement in ELECTRONICS Australia, November, 1975 on page 95 (PDF page 97) depicts M-5 series. The design is similar to that of T-5 series, and the operating cam mentioned appears to be the same T-bar actuator as found in T-5 switches.