That episode is "The Beast from 20 Fathoms" aka "Midoro Swamp". The episode was known as "lost" because for awhile no Japanese version could be found, and the only copy known to exist was the English version.
The original Japanese soudtrack has been found and it is included as a special feature in one of the Ultra collector's editions of the 1960s DVD box set. (I believe it's included in box set #2.)
The story, as explained in the box set is that Dr. Tezuka had commissioned a group of young anime artists to adapt his story, "Midoro Swamp". Tezuka is said to have hated the job done by these young professionals that he had all copies of the episode (or so he thought) destroyed!
When the search began to preserve all the episodes of 1960s Astro Boy on DVD, the Japanese soudtracks were found. I believe in some vault somewhere!
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