[Vigilanteman]

Smells like BS. The Japanese are no slouches when it comes to precision engineering.

There is a picture posted here of an open trap door in a modern Japanese prison gallows. It appears to be considerably more than 18 inches. I won't reproduce it since it is an AP link.

[Myrddin]

Detailed description of the hanging chamber with pictures at the link. More than an 18 inch drop.