So I've finished rereading the first one. Your right! It's head is "generic human". The joints for arms and legs are a mix of organic and nonorganic, but to pass, it needs a long shirt and trousers. Less human in looks than in the TV series, who we see in the nude to make it clear he is not a sex bot.

Edit:

In the second book: "I don't know if there are any augmented human with enough implants to resemble a SecUnit. It seams unlikely a human would want that many implants, or would survive whatever catastrophic injury might make them necessary."

So yes, the TV series has the body wrong even head is acceptable.