That do

[Xinfu]

p.400
-we know you're a girl,' he said, 'That do? Will you come now?'

Why do not does? Even does would be unintelligible.

[Rusty]

The phrase you question is most often said, "Will that do?"
With the modal, it should make perfect sense to you.

[Xinfu]

Ah! Good answer. Thank you.