Found this.

THE ALLURE OF DIAMONDS

The sand eel deal for jigs both in the surf and from the deck include these proven options for heavy metal inshore mayhem.
By Allen D. Riley

Bridgeport Lures of Connecticut was the first company to mass-produce diamond jigs. Their versions had four sides and were nearly square in shape when viewed from top-to-bottom. They were equipped with a treble hook attached right to the jig.

The Ava Lure Company later improved on this early design by flattening the profile, giving the lure more of a diamond shape. Instead of a treble hook attached to the jig, the Ava diamond jigs featured a swivel and a single hook which resulted in better hook-ups and allowed for easier hook removals.

Anglers who fish in saltwater should not consider their lure collection complete unless it includes several "diamonds."
Most of Ava’s diamond jigs were tagged with an “A” label to identify the weight of the lure. Originally available in two, four, and six-ounce sizes, the jigs were labeled A27, A47, and A67. To this day, many diamond jigs are still identified with the “A” classification even though Ava is no longer in business. The name recognition has stuck like barbed steel in our angler lore.
Since Ava established the industry standard for diamond jigs, other manufacturers have marketed their own variations of the popular lure.
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