Maker - Doubl-Glo

This Valentine's an old antique, For each year-r on this day Aye sent it out wi' this request: RETURN WITHOUT DELAY! but-Yir-r so sweet, 'tis yir-rs T'keep wi' ever-ry wor-r-d upon it! 'Cause all aye want is You, Dear-r -- wi' a small DEPOSIT on it!

4 3/4" x 5 3/4"  (5 3/4" x 9 1/2" opened)
circa 1940s,  1950s
 French Fold
 made in USA
 by Doubl-Glo
 marked: T-765

My Sweet Valentine Do I love YOU? ~ You can bet your BOOTS I do!

3 1/2" x 4 1/4" (4 1/4" x 6 3/8" opened)
circa 1930s, 1940s
assymetrical french fold
by Doubl-Glo

features a white cat

Doubl-Glo is a trademark used by the Paper Novelty Company of Stamford, Connecticut. The company started out making a folded paper Christmas bell, but in 1932 began making their most famous product - their tinsel in the familiar package with Santa on it.

To My Valentine  Blowing bubbles would be more fun ~ If I found you inside of one

5 1/2" x 4" (5 1/2" x 8" opened)
1940s, 1950s
French Fold
by Doubl-Glo
made in USA
marked: 700

It is not clear when they began making greeting cards. Their focus was much more on Christmas items than on valentines. 

To My Valentine Hung out my Shingle, As you can see, Tho' I'm not a Dentist Nor M.D. ~ I'll treat YOU as my clientele And practice LOVE for quite a spell Practice with You will be perfect.

3 7/8" x 4 3/4" (4 3/4" x 7 1/2 opened)
circa 1940s, 1950s
Modified French Fold
made in USA
by Doubl-Glo
marked: P 8700-4

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Sources:

The Daily Messenger, June 7th, 1967, Canandaigua, NY, p18