This keyboard was featured in my Differences between and classifications of 122-key Model Ms comparison topic.
The Type IV 122-key Model M terminal keyboard is a version of the Type III designed to be natively compatible with PCs to serve in terminal emulation roles. Officially called the IBM Personal System/2 Host Connected Keyboard, Type IVs were relatively rare but for a long time were the only 122-key Model Ms easily capable of modern usage until Soarer's Converter became widespread. Visually, Type IVs look like Type IIIs except all IBM-branded versions had lock-light LEDs as standard and come with modular SDL connections capable of accepting an AT or PS/2 plug cable. IBM also started a practice producing Type IVs under third-party branding to companies that offer terminal emulation services, which Unicomp continues to do to this day. Unicomp also produces a version of the Type IV for consumer usage called the Unicomp PC-122. Like the Type IIIs, the 24-key function key block was only labelled with an "Fxx" nomenclature.
