The Mantis light barrage vehicle was developed on the planet of Madupopia and was used heavily by the planet's inhabitents before their world was reintigrated into the imperium in recent years. As a newly reconquered world they have had very little access to standard STC designs and are still implementing a plethora of local designs. These are slowly being replaced with more standard imperial vehicles but are currently still being fielded to increase the numbers of armoured units within under strength artillery formations.Definitely Adeptus Mechanicus!
Replace a few parts with GW pieces (Nothing would be recognisable as 40k otherwise) and your good to go.
I am completely in love with the Mantis Tank! though it would be hard to justify in a Fluff sense.