Green’s Dictionary of Slang

1. the male genitals.

1937–84		Partridge DSUE (1984) 268/1: from ca. 1895.
1966		P. Boyle At Night All Cats Are Grey 67: The Kreuger-man stretched out in a drunken stupor on the cow-house floor, his fly wide open and his credentials exhibited for all the world to see.
1977		P. Caputo Rumor of War 149: When I saw it down between my legs, all I could think of were my credentials.

2. the female genitals or underwear.

2005

B. Hare Urban Grimshaw 205: Her school dress was skimpy [...] and she couldn’t help but display her credentials.