If it's an AV-3400, it would appear to be from about 1969. I base that on it looking almost exactly like the examples here:
http://www.smecc.org/sony_av-3400__avc-3400.htm
As for what's on it, maybe it'll be a missing episode of Doctor Who or footage from the first Super Bowl or a missing episode of the Art Fleming version of Jeopardy! Until you watch it, it's like Schrodinger's Cat. It's all lost television footage until you find out what it really is.