The origin or rough timeframe for the quip about pouring piss out of a boot?

Informants for the 1965-70 DARE Survey contributed folk saying variants "about a person who seems to you very stupid" (paywalled: DARE, see JJ15a and b) in response to the lead-ins "He hasn't sense enough to _________" and "He doesn't know _________."

Many variants were contributed. In response to the "He hasn't sense enough to" query, one from a Texas informant was

Pour piss out of a boot with the directions printed on the heel.

For the same lead-in, another Texas informant contributed

Pour piss out of a boot if the directions were written on the heel and the toes cut out.

A third informant, from Oklahoma, contributed

Pour piss out of a boot, and if he did, he’d step in it.

In response to the "He doesn't know" query, a Georgia informant contributed

How to pour piss out of a boot with directions written on the heel.

The earliest variant in DARE (paywalled entry) for the generalized "pour piss out of a boot, v phr" is quoted from a 1931 PMLA (45.1305):

He couldn’t pour water outn a boot an’ the directions on the heel.

It wasn't difficult to turn up a "heel directions" variant in a story from 1921:

…they don't know enough to pour sand out of a boot if they had the directions printed on the heel.

From "Fiddler's Hatch" in Wayside Tales and Cartoons Magazine, by F.C. Robertson.

And an unembellished variant as early as 1901:

"Jud," he said, "you ain't got sense enough to pour rain-water out of a boot."

Dwellers in the Hills, Melville Davisson Post.

Although evidence in print was not readily found, it would be reasonable to suppose that both the unembellished and the embellished variants might have been established in local and regional speech before 1900.