tobben63 has the correct Scott ID for the green Gold Yuan surcharge stamp.
The cover is definitely a keeper. While the stamps are quite common, the mailer is a well-known historical figure.
The green block is Scott China #757(1947). The red block appears to be perf 13 (you will need to confirm) and if so would be Scott China #818(1948). The cover is franked with total face value of $240,000 (just prior to currency revaluation).