i heard about it for the first time on this forum earlier this yearviewtopic.php?f=13&t=14664&p=470758&hilit=foreplay#p470758
Nothing i've read about it seems even remotely 1950s Toho.. if this were in the 70s, it would be a little less far fetched but even then its way out in left field. Looking at what Toho put out in the 50s and the first half of the 60s, i really find it hard to believe.
Other than that link you point to, the only other thing i have seen is the Japanese blog post its based on.
::EDIT:: So the original Japanese blog post references this book "Toho Special Effects Unpublished Document Archive"
The book apparently is mostly about G'84 and Biollante, but their is a decent portion of the book dedicated to "non-production work" which includes:
Bride of Godzilla
Godzilla legend Asuka Fortress
Godzilla vs King Kong Continued
A Human Vapor Sequel
Frankenstein vs Godzilla
Mothra vs Bagan
Flame Man vs Invisible Man(?)