The Mk 3.
The Mk 3 model is another durable variant, this time designed to also cope with extremes of temperature and pressure, as it is usually used in a Search and Rescue capacity. It also has the ability to have its 'limbs' or 'hands' replaced with inbuilt tools dependant on need. You may also note the cooling unit built into the torso area, giving it its trademark pot-bellied appearance.
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It has extra sensory equipment built into its 'head' to help detect lifeforms in an emergency situation. As with the Mk 2, it is usually used to perform tasks that are too dangerous for humans, such as in extreme heat or high pressure environments, for example in an undersea or even vacuum rescue..
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The above example has the extra sensory and detection additions fitted to the head section. You may note the appearance of this model appears rather 'unfinished'. This is because it has previously performed in a high temperature rescue and has some tarnishing to its outer shell. A more attractive appearance would be wasted on this particular model!
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The Mk 4
The Mk 4 serves a very different function in society, being an 'Academic and Tuition' model. The design of the Mk 4 differs hugely in construction to the last two variants. It may appear similar but it is a much more delicate model where most of the cost of the unit is invested in its programming chip. This chip can be changed when needed so that a single unit can instruct a wide range of subjects.
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The Mk 4 is usually made to look as friendly and welcoming as possible in its appearance and units are often made to look like a 'professor' with the addition of 'spectacles' and big round eyes incorporated into the design.
(http://i346.photobucket.com/albums/p419/thebeggarscave/6f248289-7711-4eab-9cae-152cca954404_zpsmqeyu1so.jpg)
The above example is painted in a 'calming' blue tone and features the aforementioned 'spectacle' design and large 'eyes' added to its 'head' unit. This is a common colour scheme although, especially when tutoring younger children, the arms are often finished in a yellow colour to mimic the characters from a popular youth entertainment franchise.
(https://ksr-ugc.imgix.net/assets/017/150/299/b524d94c68c2a694182395495d70f8ba_original.png?w=639&fit=max&v=1497786098&auto=format&lossless=true&s=810392bf88ca908f314fe1342707524d)