[Hugh]

In 1965 Hong Kong was still under British Administration but there were significant financial and trade ties with the PRC. The PRC treated HK as a special zone for postal purposes. HK mail would have passed through one of the Guangdong exchange offices at inland rates.

Here's a cover from the Gaoyao County Branch of the People's Bank of China to the HK branch of the Zhejiang Xingye Bank. The HK branch was affiliated with the Zhejiang Xingye Bank [AKA National Commercial Bank, Ltd. which had been established in 1907 in Hangzhou].

There was obviously so much correspondence that the PRC's People's Bank of China made the effort to pre-print the outgoing and return addresses on the envelopes they used. A common commercial practice between banks.

   

Printed in red

Addressee:
Middle column (large characters)
浙江兴业银行香港分行
[Zhejiang Xingye Bank Hong Kong Branch]

Right-hand column (smaller characters)
香港 / 大道中十号A
[Hong Kong / Des Voeux Road Central, No. 10A]

Sender:
Left-hand columns
中国人民银行高要县支行
[People’s Bank of China, Gaoyao County Branch]
地址: 广东省肇庆市红旗二路
[Hongqi 2nd Road, Zhaoqing City, Guangdong Province]
电报挂号: 41007
[Telegraph registration number: 41007]

Less common, however, is the hand-stamp the company used on the back of the cover. It is a red-ink, rubber hand-stamp with a portrait of Mao Zedong in a military cap and under, a pine branch.

As far as I can tell, the text reads something like, 敬祝毛主席 / 萬壽無疆 [May Chairman Mao live for 10,000 years without limit]  Transcription and translation advice is welcome.

Pine, an evergreen, was a traditional symbol of longevity. 

   

When this cover was sent, November 22, 1965, the Cultural Revolution had not yet started. This image and message would be very common on letters, posters and painted on walls during the ten years of turmoil (1966-1976). Seeing this political endorsement as a hand-stamp on a commercial cover, albeit from China's official bank, in late 1965 illustrates its use during the prelude period as events and tensions built leading up to the cultural revolution which started in the following May.

Stamp:
中华人民共和国 [People’s Republic of China]
Series: Government Buildings
Great Hall of the People, Beijing
June 17, 1964
8分 [Domestic rate for <20g, used for HK], Red, without gum (as issued)
Sc. 880

One of the few stamps of this period that did not follow the earlier PRC practice of numbering and dating the stamps at the bottom.

Cancel:
Single circle CDS
I can make out ... 广 东 / 高 要 [Guangdon Province, Gaoyao (county)]
Not sure about the three characters on the bottom? Corrections and advice welcome.
Date 1965.11.22

An interesting afternoon's diversion. 

Cheers, Hugh