The casting of Japanese adult film actresses like Murakami and Ayukawa reflected a common practice in Hong Kong Category III productions of the era, bringing international adult film talent to bolster the explicit content.
The keyword string "Sex and Zen -1991- -EngSub- -Hong Kong 18-" is not random. It is a technical specification. Here is what the filenames and torrent tags usually imply: Sex and Zen -1991- -EngSub- -Hong Kong 18 -