I'd think it would be good to use the current chassis code for T1 and T3 and this as Mk as well, to be really correct and uniformly.

Typ 6X is Lupo and 6E is Lupo 3L, apparently.
https://de.m.wikipedia.org/wiki/VW_Lupo

For chassis codes I go by what is used as chassis codes in the VIN (like we do for every single engine in Hondas...).