Green’s Dictionary of Slang

upright grin n.

the vagina.

1890–1904		Farmer & Henley Sl. and Its Analogues.
1896		Farmer Vocabula Amatoria (1966) 136: Fourche, f. The female pudendum; ‘the upright wink.’.
1980		Maledicta IV:2 (Winter) 185: The anatomical relationship of the bower of bliss and its main channel is indicated in such phrases as the [...] lower mouth, the upright grin (except, traditionally, in China), a bit on a fork.

In phrases

flash the upright grin (v.)

to expose one’s vagina.

1890–1904

Farmer & Henley Sl. and Its Analogues.