As with most artists' studies, Deveneck's seem to have been dashed off fairly quickly, though some have evidence of greater effort. The latter make use of a "square brush" technique whereby each brush stroke can (and often does) indicate a plane of the subject. Manchess tends to use a square brush style, so it was Duveneck's similar handling that served as inspiration.
Below are some examples from Duveneck.
This is on display at San Francisco's de Young museum. The subject's body and clothing are depicted loosely, but the face receives a careful square brush treatment.
Hardly any square brushwork here.
But more here, especially on the subject's face and left arm.
Despite her father's disapproval, Boott wanted to marry Duveneck, and they did. The title (which might not be a formal one) suggests this was painted before they were wed.
Here is Duveneck's portrait of his wife, completed around the time she died. An account of Deveneck and Boott's relationship is here. Note that square brushwork is not evident in this finshed work.