We did much of your homework for you on this one.  First, it wasn’t the only “Rare Stone Museum” in Asia — check Thailand too.  Next, it seems not to be there any more, but we didn’t dig further to learn when it closed.  Sentosa from the start has perfectly captured Singapore’s split personality, the strengths being its weaknesses at the same time.  So, this card?  From Sentosa, showing “Waterfall of Guilin, China” in what was that museum.  The card was mailed in 1986 with two stamps, postmark, and messy message.  Grade: 1