On Fri, 23 Mar 2012 21:31:10 -0700 (PDT), Philatasaur
sharpened a crayon and wrote:

I have a few of these stamps, with what appears to be St John
Ambulance hand stamps applied over.

Any idea what they were used for, and what are they worth?

Any help is appreciated

There's no such 'official' overprint listed in the SG catalogues.
British postage stamps could be used as 'small cash', so many
companies were allowed to overprint the stamps to stop pilfering.

This -=may=- be what you have.

A group specializing in the Malaysia area would be your best hope for
a definitive answer.