I wanted to check that it really is and dug out my The Roberta Williams Anthology CD with Mystery House on it and looked further.
And yes, it has indeed been put into public domain in 1987.
This is from MYSTERY.HELLO.BAS from inside MYSTHOUS.DSK (Use e.g. CiderPress to open that .DSK-file) from The Roberta Williams Anthology CD:
"FROM A GREAT MOMENT IN HISTORY:
MYSTERY HOUSE, THE FIRST GRAPHIC ADVENTURE,
WAS CREATED IN 1980 BY SIERRA ON-LINE FOUNDERS KEN AND ROBERTA
WILLIAMS, AND RELEASED INTO THE PUBLIC DOMAIN IN 1987 TO
CELEBRATE SIERRA'S 7TH ANNIVERSARY.
WE ENCOURAGE YOU TO COPY THIS GAME, FOR YOURSELF OR TO SHARE.
IF YOU HAVE ENJOYED IT YOU MAY SEND A $5 DONATION TOKEN AND
ROBERTA'S FAVORITE CHARITY, SIERRA HISTORIC SITES ASSOCIATION,
P.O. BOX 451, OAKHURST CA 93644
NOTE: BECAUSE IT IS NOW FREE, SIERRA'S GIFT TO YOU,
WE CAN NO LONGER TAKE CALLS ON THIS GAME. IF YOU NEED HELP,
WE SUGGEST YOU CONTACT YOUR LOCAL APPLE USERS GROUP TO TALK
WITH OTHERS WHO MAY HAVE PLAYED IT."
So there it is, I uploaded the MYSTHOUS.DSK to DropBox and you can get it using this link.
If anyone feels a nostalgic urge to look into adding support for Mystery House to ScummVM
now they at least have the public domain data for it.


Kari Salminen / Buddha^