Green’s Dictionary of Slang

upright grin n.

also upright wink

the vagina.

[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Farmer Vocabula Amatoria (1966) 136: Fourche, f. The female pudendum; ‘the upright wink.’.
[US]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