Need To Identify An Overprint On Chinese Revenue Stamps

I respectfully request assistance with the below pictured Chinese revenue stamps affixed to this document...





I have searched the Web and was able to locate the below information so far but am hoping someone can help with the overprint...

Republic of China issued these National General Revenue stamps from 1934, with a design picturing a Pagoda.

These set of stamps were published with same figure and different secret marks by many publishing factories.

These three (3) green 2 cents stamps were printed by JingHua Publishing House of ChongQing.

This publishing house issued these in a set of 8 stamps in the denominations of 1 cent, 2 cents, 4 cents, 6 cents, 10 cents, 20 cents, 50 cents and 100 cents.

However this information references solely the stamps themselves.

I have been seeking the significance of the overprint upon these stamps.

Can anyone here at this fine forum enlighten me on this aspect for which I am currently stymied on.

Regards,
Stampalotapus

The overprints read 'Shanghai Special Administrative District' and they are surcharged 'Special 1'.

Thank you very much tonymacg.

Regards,
Stampalotapus