Josef Pospichal lists two #21s on the roster: the one from Innsbruk and another one from Mödling-Hinterbrühl, builder Graz, 1903.

It's great that you were able to find this out so quickly: thank you! I am updating the database. The vehicle is from a system that was closed almost 90 years ago now and it happened to be preserved. Hopefully one day this tram will be publicly accessible.