[Bill]

I left Ferranti Gem Mill in 1974, at the time we were making quite a lot of 414s. Once the device per wafer yield was satisfactory, these became very cheap to make, comparable to small signal transistors. By comparison, eline plastic eg ZTX300 could be made for a penny each. I'm not sure what the applications for 414s were aimed at, may have been military, the data sheet mentions radio control receiver with very low power consumption which may have use in some sort of shell or missile? We would not be party to that information.
On the other hand, there were a lot of electronics enthusiasts at Ferranti at all levels so they may have developed it for hobbyists initially!
There was an "Applications Department" whose sole job was finding uses for new devices and publishing construction details. There were some very good HiFi transistor amp designs from there.