The only thing I have been able to discover about this uncataloged cachet is that the letters "hhc", found near the bottom of the cover, stand for Hollis Hobby Center.  The cachet features a stethoscope and a $100.00 check made out to the American Cancer Society.  The cover also includes a block of four stamps rather than the normal single stamp.  If you can provide any other information about this cachet, please drop me a line at smith@in.net.