Hirose Cherry extended B70-4753
This is a Japanese-made B70-4753 (a model that appears to have been a Cherry reference keyboard) with a curious extension attached to it with extra keys. Despite being Japanese-made, the entire unit is labelled in English.
The extension area uses extremely rare Style B′ M5 or M6 switches. The B70-4753 itself uses either Style B or B′: it is impossible to determine which it is from the photos, as you cannot see the bottom of the main area switches or the tops of the extension area switches.
The serial number appears to date it to March 1985.
These photos are from a personal listing at 闲鱼.淘宝二手. It seems impossible to view a non-mobile version of the listing without an account, but the difficult-to-follow mobile listing is id=583473125505 and available publicly (this is the page that UncleFan shared with me). As these images are sure to be deleted before too long, they are preserved here for their importance in being the first-ever discovered example of Style B′ switches.