I've never even heard of that name, Muskin.
My advice, is purchase a stick of RAM with a common name, maybe Muskin is fine and dandy at 133 but isn't truly backwards compatible with lower clocks.
Check the sites to see if they mention backward compatibility, sometimes they do and sometimes they don't even if it is.
A website that costs a little more but pays attention to details like that is
Dalco Electronics
Have had nothing but good experience with them and their memory. Have used 133 in slower before. Try Kingston, Crucial, etc.