The old Stefano & Casper site that used to identify unknown stamps is now dead, apparently - Google still archives their pages and images, and I was able to come up with this scrap of an image.
Your stamp matches stamp A, which was identified on the Stefano & Casper site as a Kwangtung postal savings stamp. Kim was correct, an excellent memory! (I can't remember where to find any of these overprints the next day after looking them up.)
The text, originally posted by Paul Luchter:
These are all Nationalist China wartime Postal Savings stamps, 1943. These seem to have been produced at first as Postal Savings stamps to encourage thrift and fight inflation, but due to inflation there was little acceptance of them by the populace. They then became a compulsory savings program when one bought luxuries like liquor or cigarettes. The stamps could be collected and turned in to banks and post offices for a savings bond. But due to inflation the savings bonds were worthless, thus this can be considered a revenue stamp dressed in postal savings stamps/bonds clothing. (There had been true postal savings stamps through the 1930's and early 1940's, first and second issues). If cancelled it would be a philatelic favor cancel. They have also never been seen on a piece of luxury item, so were evidently sold separately in some proportion to the purchase. "The low denominations and lack of odd denominations seem to support this theory" (from "The Wartime Postal Savings stamps of 1943").
Stamp A is a Postal Savings stamp 1943 for Kwangtung province, the four characters (Chu, Chieh, Chin, Chien) roughly translate as "Thrift-National Reconstruction Savings".
Stamp B is a Postal Savings stamp for Kiangsi province, 1943.
Stamp C is a Postal Savings stamp for Fukien province, the extra surcharge at bottom for 30¢ (over 10¢), 1943.
Stamp D is for Kansu, #6. It is rarer than the others, 1943.