Ledges of this type are widespread in cap merchandising, and all exhibit diagonal corners to retain the item. But this one is of Plug-in configuration.

For other means of serving-up headgear see…
Selling Caps Two-at-a-Time
Rough Cut Wood Hat Tray
Hat Ledge Plugs In
Hat Ledge for Slatwall
Hat Ledge for Slot
Built-in Mirror Sells Caps
Slatwall or Tower Hat Tray
J-Hooks Product Stop Caps
Branded Slot-Mount Hat Tray
Slotted Faux Surface for Cap Merchandising.”

For unusual cap merchandising SEE…
Gucci Gangsta Cap Display
Colorful Kangol Cap vs Slouch Hat
Winter Cap Gets the Finger.”

For headform core fixture see…
Classic Millinery Faceout
Curved Slot-Mount Cap Displayer
Slatwall Hat Displayer
L’arbre de Chapeau (Hat Tree)
Frankenstien-Inspired Fixture.”
Animal Cap Show-and-Tell
Hybrid Helmet Hook
Corrugated Headform Cut-Out

Follow the thread of plug-ins at…
Plug ‘N Play Jewelry Display
Wood Inlay as Point-of-Purchase Display
Basketball as a Seasonal Theme
Tree-Like Display Sports Metal Faceouts
Hat Ledge Plugs In
When to Use a Permanent Hook
Baseline Post for Plug-in Hooks
Plug-in and Pegboard Hooks Combine
Plug-in Hook of a Different Sort

For interesting alternatives see…
Bayonet Mount Outfits Acrylic
Bar Outfitted with Bayonet Mount
Bayonet Mount Hook
Bar Merchandiser Plugs-In
Large-Bore Plug-in Hook
Hidden Hex-Key Mount Faceout