Book Pockets and Date Guides: The Intricacies of a Paper-Based Library System – Smithsonian Libraries and Archives

https://www.indiamart.com/proddetail/charging-tray-for-library-readers-tickets-14436111112.html

A wooden 5 tray charging tray with lid! This could make a fascinating portable zettelkasten. It has options for both 2"x3" and 3"x5"Cards.

https://www.carrmclean.ca/charging-trays-and-guides.html

http://www.shopbrodart.com/Library-School-Furniture/Adult-and-Teen-Furniture/Computer-Furniture/Card-Catalog-Trays-and-Cabinets/_/Brodart-Wood-Charging-Trays2/

Brodart Full-Length Single Charging Tray<br /> Full- length charging tray with 1,000-card capacity<br /> Price: $96.32

• Adjustable steel follower block with automatic lock
• Felt pads on tray bottom protect desktop
• Full-length charging tray for countertop use
• 4"H x 4"W x 16"D
• Holds 1,000 5"H x 3"W cards
• Includes antimicrobial finish
• Made in the USA

See also: https://hypothes.is/a/ao89RMQmEe2zIvsu3lf6kw for a smaller version

http://www.shopbrodart.com/Library-School-Furniture/Adult-and-Teen-Furniture/Computer-Furniture/Card-Catalog-Trays-and-Cabinets/_/Brodart-Wood-Charging-Trays/?q=tray&s=MToyNTY6NDo6Ojo6OjA6

Brodart Mini Single Charging Tray Mini single charging tray with 600-card capacity More Info Price: $76.76

• Adjustable steel follower block with automatic lock
• Felt pads on tray bottom protect desktop
• Mini charging tray fits on your lap
• 4"H x 4"W x 8"D
• Holds 600 5"H x 3"W cards
• Includes antimicrobial finish
• Made in the USA

This could be used for a modern day Memindex box for portrait oriented 3 x 5" index cards.