America, France, French Indo-China, Germany, Italy, Japan and Russia issued stamps were (with just a few exceptions) overprinted specifically for use at their Postal Agencies on Chinese soil. Such stamps were generally for the convenience of foreign persons wishing to send international mail via their own or another postal authority, and of course to advertise their official presence in China in stamp albums throughout the world.
The Italians had the smallest international presence and some of their stamps rank amongst the rarest Foreign PO issues, perhaps the rarest being the 1917 40c on 1L brown and green which is valued by Stanley Gibbons in their 2015 catalogue at £300,000 in mint condition. The largest selection of issued stamps was probably the ranges produced by the Indo- Chinese POs in China with over 600 different basic stamps overprinted for use at various Post Offices (if you can’t find any here, check under our ‘French Colonies’ section!)
These stamps, chiefly issued between the 1880s and 1920s speak of a time where China struggled with foreign powers on her doorstep and are an integral part of a serious collection of Chinese stamps, or of colonial issues.