There are 26 episodes of OPERATION: MYSTERY (1968). Episode 24 has been recently "banned", and while included in previous VHS and Laserdisc releases, it is not included in the remastered DVD release. The episode dealt with insanity, and is now considered politically incorrect.
But, Episode 24 is up on YouTube:
http://www.youtube.com/watch?v=rphMTeQYUZM
Here's the official page for the Japanese DVD release:
http://www.dus.jp/digital_ultra_series/kaiki/index.html
The last show that Tsuburaya was involved with was CHIBIRA-KUN (1970), a series of ten-minute episodes about a family of cute monsters (think Sidmund and the Sea Monsters meet HR Puff'n'stuff), which was part of a morning children's show.
LIL' CHIBIRA:
http://www.youtube.com/watch?v=cmhDftKW_4A
The last great series that Tsuburaya produced, hands on, was HORROR THEATER: UNBALANCE. While the series was produced starting in 1969, it was not broadcast in Japan until 1973. Only 13 one-hour episodes of horror and suspense were produced, and all of them are now available on beautifully remastered DVDs.
Official UNBALANCE DVD page:
http://www.dus.jp/digital_ultra_series/ ... index.html
Here's the opening and closing for UNBALANCE (music by Isao Tomita):
http://www.youtube.com/watch?v=b20xLvA8osg
Hope this helps answer your questions!