The Theseus light tank was into production from 1934, but it did had a serious drawback : due to the shudder while the vehicle was on the move, it was impossible to fire the gun. Even at very low speeds accuracy was poor. With some improvements in suspension the accuracy problem did reduced a bit, but not enough. Another drawback of Theseus I was its riveted construction. However, Italian tanks were more or less the same, so Theseus continued in production with a new gun and turret. Late production Theseus were however welded instead of riveted.
German tanks being developed in mid 1930s (Panzer III, IV) were much better overall from Italian ones and German rearmament would possibly lead to a German attack from north, like in WW1, when Imperial Germany assisted Austro-Hungarian Empire. Another enemy from WW1 was emerged slowly but steady, Bulgaria. As a result, something better from Vickers Light Tank/Theseus was needed. This was found to the Chehoslovakian TNH series, better known with its German designation, Panzer 38(t). For license production in Hellenic Kingdom a new factory was set up in Iolkos city in late 1937. But some months later, Chehoslovakia was conquered by Germany and the team of Chehoslovak engineers who came to assist could not return to Chehoslovakia again. Hellenic government managed to transfer their families to Hellas and those engineers lived in Iolkos at least until the end of WW2.
First 15 tanks came from Chehoslovakia, then another 13 were ready or could be assembled from with the original Praga petrol engine, known as LTG* (later designated Leonidas I) . Production stopped for some months to modify the LTG for 100% local manufacturing. Most significant change was the replacement of Praga petrol with the largest TSM diesel available , the KM6. It's size was very close to Praga, power was a bit more, 160 hp at lower 2,200 rpm. Gearbox was therefore adapted to that figure to retain speed. The TSM KM type diesel was also available in 3,4,6 cylinders in line, U8, U12 and in limited number U16 (two V8 bolt together) developed for marine applications with mind. The "new" tank was renamed Leonidas from the famous Spartan King killed in Battle of Thermopylae. The only external difference was the exhaust which was curved backwards. Like all TSM diesels, the KM6 was quite smokey during acceleration under full power and the curved exhaust hide somewhat that. TSM engines were manufactured during WW2 from around 9 machine shops/factories plus TSM itself. The LTG/Leonidas was also the first welded tank manufacturered in Hellenic Kingdom.
The Leonidas was evolved to several variants through the war. First was a variation with turret from Theseus with the Chehoslovakian also 47mm, creating a better platform capable of more accurate fire. Then a completely new turret was designed, inspired somewhat from Panzer III and IV with a 57mm gun, derived from the old Hotchkiss 6-pdr (57mm/58 cal) .That fired new ammunition developed from Pyrkal based on the experience with the 47mm Chehoslovakian rounds. A supercharged version of TSM KM6, the KM6-2S1 was introduced due to heavier weight, good for 210 hp. Other early variations were command, communications (some were capable of detection and jamming enemy signals) and ambulance. An artillery tractor as also developed for the British 5,5in field gun (designated Πεδινό Πυροβόλο 140/30 Y1938 for Hellenic Army). In late 1942 the 57mm Y1940 gun was modified to 1940/42 version, which was capable for firing British 6-bdr ammunition. Finally a tank destroyer was developed on Leonidas chassis, similar with German Jagdpanzer 38, with 57mm gun or the 75mm/L47 Y1943, derived from a French AA gun or 1930's. This variant was powered by the enhanced KM6-S2, 230 hp.
*In Czech Greece is called "Řecko" and in Slovak "Grécko". So LTG is Light Tank Greece. I chose the slovak word.