Found it! At the The Lester S. Levy Collection of Sheet Music -

SAN ANTONIO
Words, Harry Williams. Music, Egbert Van Alstyne. 1907.

1. Just as the moon was peeping o'er the hill,
After the work was through,
There sat a cowboy and his partner Bill.
Cowboy was feeling blue.
Bill says, "Come down, Pal,
Down into town, Pal.
Big time for me and you.
Don't mind your old gal. You know it's cold, pal,
If what you say is true."
"Where is she now?" Bill cried,
And his partner just replied:

CHORUS: San Antoni Antonio.
She hopped up on a pony, and ran away with Tony,
If you see her, just let me know,
And I'll meet you in San Antonio.

2. You know that pony that she rode away?
That horse belongs to me.
So do the trinkets that she stow'd away.
I was the big mark E.
I won't resent it.
I might have spent it
Plunging with Faro Jack.
If she's not happy there with her chappie,
Tell her I'll take her back.
No tenderfoot like him
Could love her like her boy Jim. CHORUS

[The sheet music shows only the copyright date, not the actual date of printing. The Levy collection has 3 editions of the sheet music, so it looks as if the song was revived a couple of times after its first appearance:

This copy shows the cover only. It's rather plain. It's marked "As Produced in Mr. B.C. Whitney's 'Isle of Bong Bong' ".

This copy is complete, and it has a nice painting on the cover.

This copy has only the chorus. It's marked "As Sung and Featured by Ethel Levey," and bears a rather appealing photograph of her in a western costume.

AMG - All Music Guide lists one recent reissue of a recording by Billy Murray on the various-artists compilation "1907: Dear Old Golden Rule Days," Archeophone CD 9008, 2003. Click here to play a RealPlayer file.

The tune was used in a Bugs Bunny cartoon. More info on this page.]

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Notice sent to requestor.
-Joe Offer-