The Olivetti TES 501/55 keyboard uses a very strange design of switch. The keys are grouped into modules, but the sensing components are external to these assemblies. The only visible date is the photographs is on a National Semiconductor chip marked “MM5747CBH/N” or “MM5747C8H/N”. MM5746 is a model of National Semiconductor MOS keyboard encoder which would allow for the unidentified MM5747 to be a related part. The date code for that chip is “817”, giving us 1968, 1978 or 1988. 1978 seems to be the most probable, as 1988 is too late and 1968 is too early for that kind of encoder.
Some Olivetti keyboards were contract manufactured by other firms. For example, there is an unspecified keyboard with Alps-made snap action switches and Alps PCB code 11KF013A.
The Olivetti Praxis 20 has been found with an unidentified SMK conductive rubber dome over membrane arrangement.
Typewriter Olivetti ET 121 has been found with keyboards made by two third-party manufacturers, Alps and SMK. Both Alps and SMK manufactured identical (colour notwithstanding) elastic contact switches. PCB code 11KE086A has been found in an Alps-made keyboard, and SMK PCB code J-M6301-01Ⓓ on an SMK-made example.
There is another ET 121 variant with Alps-made snap action switches; the PCB code for this type was not investigated.