Nice find! In some of the more technical documentation they produced, IBM would refer to them as "electronic keyboards", or "electronic capacitively-coupled keyboards", the term "beam spring" has only ever been seen in patents and technical disclosure bulletins. Even then, the terminology was inconsistent, for example, some of the hardware reference manuals for systems with beam spring keyboards refer to the main element as the 'flat spring', which in the context of replacing the fly plate, kind of makes sense (i.e. the beam spring consists of flat spring + fly plate spring + fly plate)
The term "buckling spring" only really started to appear in marketing materials in the 1990s when they were selling keyboards into the open market, and customers had more choice as to the sort of keyboards that they would use with their IBM systems. The term "Selectric Touch" was also quite popular in their marketing materials.
