Battleship? Battlecruiser? Those are the two terms we have typically come to use when differentiating between 122-key Model Ms. However, as some may know, there are quite a few nuances that get overlooked when using such a 'binary' classification, which in my opinion, groups the similar-looking but measurably different keyboards together too broadly. When it came time to design my database, I settled on a four-type classification system to group the major Model "M122" variants in a way that I hope will provide a still-simple yet more accurate means to identify specific M122s. This page describes the four types in good detail and with some helpful photos.
Although, please keep in mind exceptions to rules may exist (IBM made a lot of variation amongst its products), so please only use this as a quick reference and always be sure to do your research into the specific part number if you're still not sure. Also please note that these types are not a replacement/alternative to 'generations', since the latter describes the production quality (such as the keyboard assembly's backplate thickness) rather than feature compliment. Some of these types were in fact produced across different generations, with such differences usually obscured from an external visual standpoint.
The first type of M122 was perhaps the most conservatively designed Model Ms ever. Type Is look virtually identical to the 122-key Model Fs they succeeded when looking from a top, front, rear or side perspective, with the only visual distinction between them being the material and colour of the bottom casing the keyboard assembly sits on. The F122s had a black metal 'pan', whereas Type Is have white plastic. As such, they retained the two-setting riser-style feet and metal-jacketed 240-degree DIN connector.
The second type of M122 looks very similar to the first one, except there are more visual cues to distinguish them from F122s. For starters, gone are the side-accessible riser feet settings - Type IIs have Model M-style flip-out feet that are underside accessible. On average, Types II typically lack stepped keycaps where the PC keyboard modifiers would usually reside and the DIP switch panel underneath is typically unpopulated. Although the best way to tell a Type II from a glance is the connector - the straight metal-jacketed DIN plug is replaced with a plastic-jacketed 90-degree angled plug.
The third type of M122 features the most radical changes between the different types. They are instantly and easily identifiable since they feature a completely different design language that's distinctly Model M (as opposed to being a mere internal rework of an F122). The Type III features slimmer bezels, smaller surface area around the 24-key function block, exclusively features the said 24-key block with "Fxx" numbering scheme as standard, and adds a three-way cable router to the underside. The connector is also very different, being a modular 8P5C design (ie, same physical jack as RJ-45/ethernet). And finally, in line with pretty much all Model Ms from 1987 and onwards, the silver square IBM logo was swapped for oval-style logos; originally with grey IBM text, later (1992 onwards) with blue text.
The fourth type of M122 is simply the PC-compatible version of Type III, however, these are still easily distinguishable due to the presence of lock-lights and the modular SDL connector on the back that could accept either 180-degree DIN (AT-style) or mini-DIN (PS/2) cables. Another remarkable feature is the presence of dual dye-sublimated legends, which feature the terminal orientated legends as the primary black-coloured legends with a subset of PC-centric legends as the secondary blue-coloured legends. According to IBM literature, these were also officially known as PS/2 Host Connected Keyboards.
Unicomp continues to make keyboards based on the Type IV design for companies that require IBM 3270 or 5250 terminal emulation or consumers wishing a 122-key fully-PC keyboard. They are not much different compared to Type IV proper (hence the lack of distinct classification, for the moment), but there are some nuances worth mentioning; cables are fixed once more, some companies choose to cover the lock-lights with a blanking plate, and black-coloured casing, USB and even a magnetic stripe card attachment are now available as options. Unicomp also allows customers to customise the firmware somewhat too, so two given examples of the seemingly same keyboards may behave differently. Unicomp's marketing name for these keyboards is PC-122.