Early this spring, I was contacted by Mr. Jozef Wagemakers of Holland. He is updating the list of 'Bilingual Cancels' from Mr. Paul Davey of England and would like to get some information from my collection on the topic.
After a few email with both Mr. Wagemakers & Mr. Davey, I was side tracked by a very interesting aspect of the Bilingual cancels and that is the Chinese characters replacement of the year plug used in Shanghai.

The above is an example used in 1904 from Shantung Huanghsien to Yangchow with a Chefoo Bilingual transit cancel of  2? FEB 04 and the Shanghai Bilingual cancel of 25 (inverted) FEB "CHEN". Another example below,

is on a cover from Soochow to Kaiting with a Soochow Dollarchop cancels of 8 FEB 04, red Soochow letter box 8 tombstone cancel, kaiting tombstone and the Shanghai Bilingual cancel of  9 FEB 'CHEN'. The only two examples in my collection.
The type of cancels is rated by Mr. Davey as 'rare' and according to Mr. Davey and Mr. Wagemakers four 'Horary' characters were found replacing the year plug and they are "CHEN"  , "I"  , "WU"   and "WEI"   . Mr. Davey have one example and Mr. Wagemakers have four.
Mr. Paul Chang recorded "I" , "WU" and "WEI" , in page 252 and 258 of his book "History of Postal Cancellation of China" Part II and also illustrated the "I" and "WU" character cancellation and stated that the "I" cancel is on a cover from Shanghai to France in 1910.
In page 91of the book "The Cancellations of Imperial China" by Sun K. Y.,   a "WEI" character cancellation is illustrated and Mr. Sun stated that the cancel was on a 1904 cover.
I then did a paper search on all my books and auction catalogs and found two more examples. An example of "CHEN" is found on the MAY 1995 Sotheby's auction lot 1616 used on a 1904 cover.   The other example, to my surprise is found in Mr. Paul Chang's book "History of Postal Cancellation of China" Part III image 16-12-2 with a "WEI" cancels used in 1904 which Mr. Chang didn't realized that he have the example.
As to the reasons why the character was used? It is suggested that either it was used at sub-offices; for time of the day or even international and express mail, but no one could be certain until more covers are found.
The reason as to why not much is known on these cancellations is due to the facts that the characters is poorly defined and  that few people know of it existence and also it is not a common cancel.
So look at your collection again and you might find a couple of these examples.

Below is an example of "WU" from Mr. Paul Davey.
Below are examples of a "CHEN" , two "I"  and a "WU" from Mr. Josef Wagemakers.