I keep hearing of the original design for the Russ being based on a tractor or some other utilitarian vehicle turned war machine. I can't really believe it overall at least not in a manner of the Leman Russ being a tractor with a gun bolted on.
Mostly due to the rhomboid tracks.
It might be my knowledge of history with the early tanks literally taking the tracks and suspension designs from the Holt Tractor and just bolting armored boxes on top of these tracks (see the French
St Chamond and the German A7V).
While others divert completely the other way and practically ignore the tractor altogether, only keeping the tracks themselves and making a new suspension design like the British rhomboid tanks, the Whippet "medium" tank, the
FT-17 Renault etc.
by interwar tanks barely had any influence of the Holt tractor left. and when WW2 hit there was no trace left besides the constant caterpillar tracks themselves which had improved and changed in design.
My personal beliefs of the Leman Russ origins is a hybrid of WW1 tank creation and the evolution of the MBT.
It could have been influenced by the original utility vehicle (the missing [track] link) but the design quickly gained the rhomboid appearance of its brethren (like the forge world heresy tanks) and over time it was constantly refined and perfected, much like the lascannon. as it evolved from the Mk1 its turret widened and flattened as it progressed through the marks and with combat experience and the introduction of a modular turret design that is roughly standardized in the imperium, multiple variants evolved with different weapons systems and abilitys.
The modular set up of the Russ, its sheer robustness, ability to run on anything that burns, and the fact that an idiot could operate the machine and it would keep going made it outlive its mechanical brothers in arms not counting its ancient big brother the Baneblade which possibly influenced the little Russ due its modular design and ability to have its hull or other systems calibrated to be used for other roles like Titan Killer, Siege breaking and more.
Its kind of how I came to the design concept of my own version of the Russ, which I named the "Vanguard" but that's another story.
As for "official" history. All I can find it that it says it served in the Great Crusade (battle tank and executioner versions) and that "all other history is missing" or some other wording saying there is no information on where it came from.
Edit: I found a tank that looks a lot like our little friend the Russ.
The French ARL 44 is a WW2 tank design (designed in ww2, and actually built afterwards)
It looks a lot like the Russ, make the tracks likely more rhomboid, bolt on sponsons and a lascannon and your really close.