T stands for Tikkakoski , designing the producer of this ammo: Oy Tikkakoski Ab. The factory was located in the Tikkakoski district in Jyväskylä, Central Finland .
41 stands for 1941 the date of production
20 is the caliber 20mm
/40 designates the type of the gun (or rather model of the case): "model 40" (though designed in 1939 First pre-production series of the L-39 was finished on April 1940 )
Here a similar cartridge and markings, the only difference is the E 4 instead of 5, probably a production batch designation.
and one without the supposed Batch code
Here are similar markings on a cartridge of 20 x 138 B Rheinmetall Flak 30/38 produced by this same company
and on a cartridge of the Lahti-Tikkakoski antitank 20x113
Note that there was an evolution in the markings according to production . Here for instance the 3 kinds of marking used by the same company on the 20x113 Lathi L-34 anti-tank, in accordance to the series/type produced:
Left: 135g Solid AP(Practice/Unhardened steel)
Middle: 150g AP-T(New and old colorings)
Right: "Italian" 136g HE-T(SD) (Finnish produced cartridges loaded with Italian HE projectiles)
