I saw this four part mini series a few times during the early 1970s in my native Germany. The first time still on a black-and-white TV set.
I remember being all excited about it and knowing that it was not a US production (even though it took place in the US), but still I loved every minute of it since it was about distant places, memories and adventures - so far away from the day-to-day life in Central Europe back then.
Watching it now it seems rather dated. The low budget shows, filming took place in Romania at the river Donau, sets are rather small (Aunt Polly's house has - poorly - painted backgrounds) and even though done with love care and craftsmaship, the only existing river steam boat (standing in for a few) is a boat dressed up to look vintage American a "side wheeler" as known to experts. Exhaust steam coming from underneath the paddle boxes where no paddle wheels are seen (because there are no): wrong! - it would come out of steam pipes that run along with the twin funnels.
Also: the German language version sounds really dated by now. There is an old fashioned "1930s Berlin low life backyard language" used by Tom Sawyer's voice and the Narrator somehow sounds too "preachy" and "patronizing" (wherweas Huck Finn sounds too educated)- perhaps that already was evident back when the movie first was aired - even though a quite a lot of dialog has been lifted~literally out of the German issues of the Mark Twain stories.
Still a classic - to be treasured - that brings back fond memories to people who were children back then, but it definitely is dated by now.
